Fix duplicate keybinding in the MPD keymap

"S" was first bound to MPD-CURRENT-SONG and then to *MPD-SEARCH-MAP*. As
a result only *MPD-SEARCH-MAP* was accessible. The key "i" was still
free, so we bind that to MPD-CURRENT-SONG instead.

Fixes #46

minor-mode/mpd/mpd.lisp

@@ -975,7 +975,7 @@ Passed an argument of zero and if crossfade is on, toggles crossfade off."
              (kbd "r") "mpd-toggle-repeat"
              (kbd "f") "mpd-toggle-xfade"
              (kbd "F") "mpd-set-xfade"
-             (kbd "S") "mpd-current-song"
+             (kbd "i") "mpd-current-song"
              (kbd "p") "mpd-play"
              (kbd "q") "mpd-browse-playlist"
              (kbd "o") "mpd-stop"