[class.union.general] Add comma (#7072)

Author: Andreas-Krug

source/classes.tex

@@ -3069,7 +3069,7 @@
 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
-reference type the program is ill-formed.
+reference type, the program is ill-formed.
 \begin{note}
 Absent default member initializers\iref{class.mem},
 if any non-static data member of a union has a non-trivial