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.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.