vit: Fix installation of vit-commands help file

vit-commands is installed into 'etc', but it needs to go in
`$prefix/etc`, not in `$HOMEBREW_PREFIX/etc`. It's not actually a 
configuration file, and vit looks for it in its keg prefix.

Ideally the file wouldn't even be symlinked into `$HOMEBREW_PREFIX/etc` but I
don't see any way to avoid that.