Update openmw.desktop

Fix bad output of desktop-file-validate for openmw.dektop. This command return, on the old file:
openmw.desktop: error: value "Morrowind;Reimplementation Mods;esm;bsa" for locale string list key "Keywords" in group "Desktop Entry" does not have a semicolon (';') as trailing character
So I added a ";" as trailing character, and this work fine. (I send the same request for opencs.desktop)