Made changes to preface.asciidoc

preface.asciidoc

@@ -52,7 +52,7 @@ All the code snippets can be replicated on most operating systems with a minimal
 
 Some of the code snippets and code output have been reformatted for print. In all such cases, the lines have been split by a backslash (\) character, followed by a newline character. When transcribing the examples, remove those two characters and join the lines again and you should see identical results as shown in the example. 
 
-All the code snippets use real values and calculations where possible, so that you can build from example to example and see the same results in any code you write to calculate the same values. For example, the private keys and corresponding public keys and addresses are all real. The sample transactions, blocks, and block chain references have all been introduced in the actual bitcoin block chain and are part of the public ledger, so you can review them on any bitcoin system.
+All the code snippets use real values and calculations where possible, so that you can build from example to example and see the same results in any code you write to calculate the same values. For example, the private keys and corresponding public keys and addresses are all real. The sample transactions, blocks, and blockchain references have all been introduced in the actual bitcoin blockchain and are part of the public ledger, so you can review them on any bitcoin system.
 
 === Using Code Examples