Update configuration - Mercurial command is hg

Although the package is called mercurial and the VCS' name is Mercurial the
command thats run (by etckeeper) is called 'hg'.
Fix syntax error in template

I hadn't tested before pushing this 'easy' change to the template which has
resulted in a failure. This updated version should work as expected.
Merge pull request #1 from goetzk/Mercurial-uses-hg-for-command

Update configuration - Mercurial command is hg