Rename countries to regions. (#465)

15.md

@@ -55,7 +55,7 @@ A merchant can publish these events:
             "id": <String, UUID of the shipping zone, generated by the merchant>,
             "name": <String (optional), zone name>,
             "cost": <float, base cost for shipping. The currency is defined at the stall level>,
-            "countries": [<String, countries included in this zone>],
+            "regions": [<String, regions included in this zone>],
         }
     ]
 }