[FIX] point_of_sale: typo in pos.config.settings (wrong model inheritance)

This fixes an error message preventing a non-system-manager user to edit the
Point of sale settings.

addons/point_of_sale/res_config.py

@@ -1,7 +1,7 @@
 from openerp.osv import fields, osv
 
 class pos_configuration(osv.TransientModel):
-    _inherit = 'base.config.settings'
+    _inherit = 'res.config.settings'
     _name = 'pos.config.settings'
 
     _columns = {