[gdb/doc] Fix "Standard Replies" xref

When building gdb with an older makeinfo (4.13), I run into:
...
gdb/doc/gdb.texinfo:42613: warning: `.' or `,' must follow @xref, not `f'.
...

This is related to this line:
...
@xref{Standard Replies} for standard error responses, and how to
respond indicating a command is not supported.
...

Fix this by adding a comma.

Tested by rebuilding the docs.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Co-Authored-By: Eli Zaretskii <eliz@gnu.org>