Update preface.asciidoc

Few spelling and grammar changes.

preface.asciidoc

@@ -19,7 +19,7 @@ _The best way to get the right answer on the Internet is not to ask a question,
 
 I hope you can help me find and publish the "right answer" by the time this book is ready to print.
 
-The early-realease source code is published on Github: https://github.com/aantonop/bitcoinbook
+The early-release source code is published on Github: https://github.com/aantonop/bitcoinbook
 
 File an "Issue", or fork, modify and create a "Pull Request" with your changes.
 
@@ -74,7 +74,7 @@ pass:[<ulink role="orm:hideurl:ital" url="http://my.safaribooksonline.com/?porta
 
 Technology professionals, software developers, web designers, and business and creative professionals use Safari Books Online as their primary resource for research, problem solving, learning, and certification training.
 
-Safari Books Online offers a range of pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/subscriptions">product mixes</ulink>] and pricing programs for pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/organizations-teams">organizations</ulink>], pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/government">government agencies</ulink>], and pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/individuals">individuals</ulink>]. Subscribers have access to thousands of books, training videos, and prepublication manuscripts in one fully searchable database from publishers like O’Reilly Media, Prentice Hall Professional, Addison-Wesley Professional, Microsoft Press, Sams, Que, Peachpit Press, Focal Press, Cisco Press, John Wiley & Sons, Syngress, Morgan Kaufmann, IBM Redbooks, Packt, Adobe Press, FT Press, Apress, Manning, New Riders, McGraw-Hill, Jones & Bartlett, Course Technology, and dozens pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/publishers">more</ulink>]. For more information about Safari Books Online, please visit us pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/">online</ulink>].
+Safari Books Online offers a range of pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/subscriptions">product mixes</ulink>] and pricing programs for pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/organizations-teams">organizations</ulink>], pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/government"> government agencies</ulink>], and pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/individuals">individuals</ulink>]. Subscribers have access to thousands of books, training videos, and prepublication manuscripts in one fully searchable database from publishers like O’Reilly Media, Prentice Hall Professional, Addison-Wesley Professional, Microsoft Press, Sams, Que, Peachpit Press, Focal Press, Cisco Press, John Wiley & Sons, Syngress, Morgan Kaufmann, IBM Redbooks, Packt, Adobe Press, FT Press, Apress, Manning, New Riders, McGraw-Hill, Jones & Bartlett, Course Technology, and dozens pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/publishers">more</ulink>]. For more information about Safari Books Online, please visit us pass:[<ulink role="orm:hideurl" url="http://www.safaribooksonline.com/">online</ulink>].
 
 === How to Contact Us
 
@@ -141,8 +141,8 @@ difficulty target::
 ((("target difficulty")))
  	A difficulty at which all the computation in the network will find blocks approximately every 10 minutes.
 
-difficulty re-targetting::
-((("difficulty re-targetting")))
+difficulty re-targeting::
+((("difficulty re-targeting")))
 	A network-wide re-calculation of the difficulty which occurs once every 2106 blocks and considers the hashing power of the previous 2106 blocks.
 	
 fees::