Better wording: "Add Separator" instead of "Begin Group"

This is the command that adds a Separator to the toolbar or menu

Change-Id: I38bb449b6f34b6fc80bbbe6c0161d3346f785888
Reviewed-on: https://gerrit.libreoffice.org/6088Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>

cui/source/customize/cfg.src

@@ -63,7 +63,7 @@ TabDialog RID_SVXDLG_CUSTOMIZE
     Text [ en-US ] = "Menu" ;\
 
 #define TEXT_BEGIN_GROUP \
-    Text [ en-US ] = "Begin a Group" ;\
+    Text [ en-US ] = "Add Separator" ;\
 
 #define TEXT_RENAME \
     Text [ en-US ] = "Rename..." ;\