Update associated record creation example

While the example is probably the intended behavior it does not reflect the current request path. I've updated the example to more closely match the current experience.

If one of the potential fixes like remi#318 is found to be acceptable it would make sense to include an update to the READ.me to reflect the changes.

Author: mascott

README.md

@@ -397,8 +397,8 @@ You can use the association methods to build new objects and save them.
 @user.comments.build(body: "Just a draft")
 # => [#<Comment body="Just a draft" user_id=1>]
 
-@user.comments.create(body: "Hello world.")
-# POST "/users/1/comments" with `body=Hello+world.`
+@user.comments.create(body: "Hello world.", user_id: 1)
+# POST "/comments" with `body=Hello+world.&user_id=1`
 # => [#<Comment id=3 body="Hello world." user_id=1>]
 ```