Fix desktop-file-validate warning.

--- a/data/gtg.desktop	2014-11-01 11:23:31.101881704 +0100
+++ b/data/gtg.desktop	2014-11-01 11:23:37.349775014 +0100
@@ -14,7 +14,7 @@
 StartupNotify=true
 X-Ayatana-Desktop-Shortcuts=NewTask
 
-[NewTask Shortcut Group]
+[X-NewTask Shortcut Group]
 Name=New Task
 Exec=gtg_new_task
 TargetEnvironment=Unity