[class.local] Add comma after introductory phrase (#6545)

Author: eisenwave

source/classes.tex

@@ -3358,7 +3358,7 @@
 
 \pnum
 \indextext{nested class!local class}%
-If class \tcode{X} is a local class a nested class \tcode{Y} may be
+If class \tcode{X} is a local class, a nested class \tcode{Y} may be
 declared in class \tcode{X} and later defined in the definition of class
 \tcode{X} or be later defined in the same scope as the definition of
 class \tcode{X}.