Variable $base_path is not set in permission_select_help()

First problem: the variable $base_path used in the help message is not set.
Second problem: this variable is insetred in a translated string.

The patch below modifies the message so the string needs only to be translated one time for any server (dev, prod...) or in case or multisites.

And instead of $base_path .'admin/build/modules', it uses a pattern replacement for !url: url('admin/build/modules').