exchange: fix issue3781 reference in the comment

This comment is about:

  issue3781: Courtesy Phases synchronisation to publishing server prevent
             subrepo push
Not about:

  issue3871: Slow hg log when template contains {file_adds}, {file_mods} and
             {file_dels}

mercurial/exchange.py

@@ -537,7 +537,7 b' def _pushdiscoveryphase(pushop):'
         # - and remote support phase
         # - and no changeset are to be pushed
         # - and remote is publishing
-        # We may be in issue 3871 case!
+        # We may be in issue 3781 case!
         # We drop the possible phase synchronisation done by
         # courtesy to publish changesets possibly locally draft
         # on the remote.