Fix command for linking modules in mods-enabled.

Fix command for linking modules in mods-enabled.

Running `ln -s mods-available/foo mods-enabled/foo` will result in a dead link: `mods-enabled/foo` will point to `mods-enabled/mods-available/foo`, which doesn't exist. The link is relative from its location, not from the current directory from which it was created.

The easiest method that allows using tab completion is to link from within `mods-enabled`. The second parameter to `ln` can be left off in that case, as well. This is the change I have proposed. Another alternative would be to run `ln -s ../mods-available/foo mods-enabled/foo` from the `raddb` directory.