Merge pull request #2085 from tricknotes/fix-doc

Fix example code in `doc/general/getting_started.md`

Author: bf4

docs/general/getting_started.md

@@ -37,7 +37,7 @@ and
 class CommentSerializer < ActiveModel::Serializer
   attributes :name, :body
 
-  belongs_to :post_id
+  belongs_to :post
 end
 ```