fix help function definition to be posix shell compliant

I let the bashism slip into this patch, which breaks it when run under
ash on ubuntu (hence failing in the devstack doc job).

http://unix.stackexchange.com/questions/73750/difference-between-function-foo-and-foo
seems to indicate that this is the syntax that will work.
Merge pull request #3 from sdague/rst_support

fix help function definition to be posix shell compliant