Add %F to drracket desktop file

Add %F to the exec key of the Dr. Racket desktop file. The parameter is explained [here] (http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#exec-variables).

Among other things, this lets Dr. Racket appear in Gnome 3's Open With dialog (at least on my system), allowing .rkt files to open by double clicking them.