Update associations.md

To support an alias for HasMany, the alias name must be specified in include, otherwise it will throw error like `unhandled rejection Error: Categories is not associated to Product!`

docs/docs/associations.md

@@ -813,7 +813,10 @@ Product.create({
     {id: 2, name: 'Beta'}
     {id: 2, name: 'Beta'}
   ]
   ]
 }, {
 }, {
-  include: [ Categories ]
+  include: [{
+    model: Categories,
+    as: 'categories'
+  }]
 })
 })
 ```
 ```