mpd: fix spelling in description

(cherry picked from commit 323c2a1c6b1fe17d7f32451ad82e646c5e610e3c)

nixos/modules/services/audio/mpd.nix

···
         defaultText = ''''${dataDir}/tag_cache'';
         description = ''
           The path to MPD's database. If set to <literal>null</literal> the
-          paramter is omitted from the configuration.
+          parameter is omitted from the configuration.
         '';
       };
     };