Remove extraneous "}" from macros documentation

Somehow this isn't a problem for Doxygen, but it's still wrong to have
unmatched "}" here.

docs/doxygen/mainpages/const_cpp.h

@@ -366,7 +366,7 @@ more details.
         includes a number of wxWidgets headers thus making it unnecessary to include
         them explicitly. However if this is not defined, you do need to include them
         and so the usual idiom which allows to support both cases is to first include
-        @c wx/wxprec.h} and then, inside <tt>\#ifndef WX_PRECOMP</tt>, individual
+        @c wx/wxprec.h and then, inside <tt>\#ifndef WX_PRECOMP</tt>, individual
         headers you need.}
 @itemdef{_UNICODE and UNICODE, are both always defined in current wxWidgets
     versions.}