desktop: use Office;Calendar; instead of Utility; categories

Reported-By: Michael Heyns <mike.bean.heyns@gmail.com>
Reference: https://bugzilla.gnome.org/show_bug.cgi?id=751571
Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>

data/org.gnome.Calendar.desktop.in.in

@@ -7,7 +7,7 @@ Terminal=false
 Type=Application
 StartupNotify=true
 OnlyShowIn=GNOME;
-Categories=GNOME;GTK;Utility;Core;
+Categories=GNOME;GTK;Office;Calendar;Core;
 _Keywords=Calendar;Event;Reminder;
 MimeType=text/calendar;
 DBusActivatable=true