update item group file location (#2418)

* Updated documentation for item groups.
Thanks to @sader1992!

doc/item_group.txt

@@ -3,7 +3,7 @@
 //===== By: ==================================================
 //= rAthena Dev Team
 //===== Last Updated: ========================================
-//= 20131223
+//= 20170915
 //===== Description: =========================================
 //= Explanation of the item_group.txt file and structure.
 //============================================================
@@ -38,7 +38,7 @@ The table below explains which fields are accessed in each.
 
 ---------------------------------------
 
-GroupID: See the "Item Group ID" section in 'db/const.txt'.
+GroupID: See the "Item Group ID" section in 'src/map/itemdb.h' and the "item groups" section in 'src/map/script_constants.h'.
          Supports IG_* constants.
 
 ---------------------------------------