Fix a typo in ch01.asciidoc: add '?'

ch01.asciidoc

@@ -32,7 +32,7 @@ In this chapter we'll get started by explaining some of the main concepts and te
 ((("bitcoin","precursors to")))The emergence of viable digital money is closely linked to developments in cryptography. This is not surprising when one considers the fundamental challenges involved with using bits to represent value that can be exchanged for goods and services. Two basic questions for anyone accepting digital money are:
 
 1.     Can I trust the money is authentic and not counterfeit?
-2.     Can I trust that the digital money can only be spent once (known as the((("double-spend problem"))) “double-spend” problem.)
+2.     Can I trust that the digital money can only be spent once (known as the((("double-spend problem"))) “double-spend” problem)?
 3.     Can I be sure that no one else can claim that this money belongs to them and not me? 
  
 ((("counterfeiting")))((("crypto-currency","counterfeiting")))Issuers of paper money are constantly battling the counterfeiting problem by using increasingly sophisticated papers and printing technology.  Physical money addresses the double-spend issue easily because the same paper note cannot be in two places at once. Of course, conventional money is also often stored and transmitted digitally. In these cases, the counterfeiting and double-spend issues are handled by clearing all electronic transactions through central authorities that have a global view of the currency in circulation. For digital money, which cannot take advantage of esoteric inks or holographic strips,((("cryptography"))) cryptography provides the basis for trusting the legitimacy of a user’s claim to value.  Specifically, cryptographic digital signatures enable a user to sign a digital asset or transaction proving the ownership of that asset. With the appropriate architecture, digital signatures also can be used to address the double-spend issue.