push: fix docsstring

Seems the code block misses `::`. This patch makes sure `[push]` and
`pushvars.server = true` are in two lines.

Differential Revision: https://phab.mercurial-scm.org/D411

mercurial/commands.py

@@ -4022,7 +4022,7 b' def push(ui, repo, dest=None, **opts):'
         strings that look like conflict markers.
 
         By default, servers will ignore `--pushvars`. To enable it add the
-        following to your configuration file
+        following to your configuration file::
 
             [push]
             pushvars.server = true