don't pass function return as reference in html_hilight()

Ignore-this: 19a8e8cbdbafea2268a31a08a65f4e05

This was actually reported [1] as a bug elsewhere, but really should be
fixed in DokuWiki and now is.

[1] https://www.projectzero.org/bugzilla/show_bug.cgi?id71

darcs-hash:20090526073013-7ad00-90c0bee83ebc6c9c3e33b2a58b8d0a17d828f748.gz