Don't encode argument passed to the debwiki shortcut.

Else, [[!debwiki Derivatives/Guidelines]] points to
https://wiki.debian.org/Derivatives%2FGuidelines (404), instead of the correct
https://wiki.debian.org/Derivatives/Guidelines.

Reported upstream:
https://ikiwiki.info/bugs/debwiki_shortcut_creates_buggy_URLs_to_subpages/