[class.union] Change "but not" to "but it shall not have" as suggested by Hubert Tong.

Author: Dawn Perchik

source/classes.tex

@@ -1239,7 +1239,7 @@
 \indextext{destructor!\idxcode{union}}%
 A union can have member functions (including constructors and destructors),
 \indextext{restriction!\idxcode{union}}%
-but not virtual~(\ref{class.virtual}) functions. A union shall not have
+but it shall not have virtual~(\ref{class.virtual}) functions. A union shall not have
 base classes. A union shall not be used as a base class.
 \indextext{restriction!\idxcode{union}}%
 If a union contains a non-static data member of