keyboard: Be more specific in input source switch label

The description of these shortcuts should mention 'input source'.
https://bugzilla.gnome.org/show_bug.cgi?id=692567

panels/keyboard/01-input-sources.xml.in

@@ -4,9 +4,9 @@
                 _name="Typing">
 
   <KeyListEntry name="switch-input-source"
-                _description="Switch to next source"/>
+                _description="Switch to next input source"/>
 
   <KeyListEntry name="switch-input-source-backward"
-                _description="Switch to previous source"/>
+                _description="Switch to previous input source"/>
 
 </KeyListEntries>