[FIX] purchase: field label

No need for uppercase, courtesy of Lionel Sausin.

Well, Clarice - have the lambs stopped screaming?
  -- Hannibal Lecter

addons/purchase/stock.py

@@ -227,7 +227,7 @@ class stock_warehouse(osv.osv):
     _columns = {
         'buy_to_resupply': fields.boolean('Purchase to resupply this warehouse', 
                                           help="When products are bought, they can be delivered to this warehouse"),
-        'buy_pull_id': fields.many2one('procurement.rule', 'BUY rule'),
+        'buy_pull_id': fields.many2one('procurement.rule', 'Buy rule'),
     }
     _defaults = {
         'buy_to_resupply': True,