Fix default MPD time format (#841)

Change the default value of the `time_format` setting in the mpd module
so that it matches the default value used by `TimeWrapper`.

This fixes a bug where the seconds weren't displaying correctly due to a
typo.

Before: `02:MS`
After:  `2:06`

i3pystatus/mpd.py

@@ -97,7 +97,7 @@ class MPD(IntervalModule):
     color_map = {}
     max_field_len = 25
     max_len = 100
-    time_format = "%M:MS"
+    time_format = "%m:%S"
     truncate_fields = ("title", "album", "artist", "album_artist")
     hide_inactive = False
     on_leftclick = "switch_playpause"