[basic.link] Add oxford comma

Author: eisenwave

source/basic.tex

@@ -2714,8 +2714,8 @@
 \pnum
 An unnamed namespace or a namespace declared directly or indirectly within an
 unnamed namespace has internal linkage. All other namespaces have external linkage.
-The name of an entity that belongs to a namespace scope
-that has not been given internal linkage above
+The name of an entity that belongs to a namespace scope,
+that has not been given internal linkage above,
 and that is the name of
 \begin{itemize}
 \item a variable; or