[basic.def] Fix punctuation (#6766)

Author: Andreas-Krug

source/basic.tex

@@ -228,7 +228,7 @@
 a \grammarterm{template-declaration}\iref{temp.pre}
 whose \grammarterm{template-head} is not followed by
 either a \grammarterm{concept-definition} or a \grammarterm{declaration}
-that defines a function, a class, a variable, or a static data member.
+that defines a function, a class, a variable, or a static data member,
 \item it is
 an explicit instantiation declaration\iref{temp.explicit}, or
 \item it is