[temp.pre] Add comma after introductory clause (#6814)

Author: Eisenwave

source/templates.tex

@@ -123,7 +123,7 @@
 \pnum
 In a
 \grammarterm{template-declaration},
-explicit specialization, or explicit instantiation the
+explicit specialization, or explicit instantiation, the
 \grammarterm{init-declarator-list}
 in the declaration shall contain at most one declarator.
 When such a declaration is used to declare a class template,