Fix the TeX compile error.

Doc/ref/ref2.tex

@@ -321,7 +321,7 @@ def       for       lambda    try
 
 Note that although the identifier \code{as} can be used as part of the
 syntax of \keyword{import} statements, it is not currently a reserved
-word by default.}
+word by default.)
 
 \versionchanged[Both \keyword{as} and \keyword{with} are only recognized
 when the \code{with_statement} feature has been enabled.  It will always