Fix punctuation detail

Just if it was not intentional.

book/book.tex

@@ -12927,7 +12927,7 @@ \section{Prototyping versus planning}
 An alternative is {\bf designed development}, in which high-level
 insight into the problem can make the programming much easier.  In
 this case, the insight is that a Time object is really a three-digit
-number in base 60 (see \url{http://en.wikipedia.org/wiki/Sexagesimal}.)!  The
+number in base 60 (see \url{http://en.wikipedia.org/wiki/Sexagesimal}).  The
 {\tt second} attribute is the ``ones column'', the {\tt minute}
 attribute is the ``sixties column'', and the {\tt hour} attribute is
 the ``thirty-six hundreds column''.