help: refer ":merge3" instead of "internal:merge3"

According to warning message (introduced by 38e0363dcbe0) in
filemerge.py, the former should be used as official name.

mercurial/help/config.txt

@@ -982,7 +982,8 b' Supported arguments:'
   launching external tool.  Options are ``true``, ``false``, ``keep`` or
   ``keep-merge3``. The ``keep`` option will leave markers in the file if the
   premerge fails. The ``keep-merge3`` will do the same but include information
-  about the base of the merge in the marker (see internal:merge3).
+  about the base of the merge in the marker (see internal :merge3 in
+  :hg:`help merge-tools`).
   Default: True
 
 ``binary``