help: format ``commands`` heading correctly

The number of dashes under it needs to match exactly for it to be
rendered as a heading. Without this change, the dashes end up on the
same line as "commands", and "hg help config.commands" does not work.

mercurial/help/config.txt

@@ -415,7 +415,7 b' effect and style see :hg:`help color`.'
     extension).
 
 ``commands``
-----------
+------------
 
 ``status.relative``
     Make paths in ``hg status`` output relative to the current directory.