Change incorrect example URL (#4261)

Changed http://example.com/api/items/4/.json to http://example.com/api/items/4.json

docs/tutorial/2-requests-and-responses.md

@@ -96,7 +96,7 @@ Notice that we're no longer explicitly tying our requests or responses to a give
 
 ## Adding optional format suffixes to our URLs
 
-To take advantage of the fact that our responses are no longer hardwired to a single content type let's add support for format suffixes to our API endpoints.  Using format suffixes gives us URLs that explicitly refer to a given format, and means our API will be able to handle URLs such as [http://example.com/api/items/4/.json][json-url].
+To take advantage of the fact that our responses are no longer hardwired to a single content type let's add support for format suffixes to our API endpoints.  Using format suffixes gives us URLs that explicitly refer to a given format, and means our API will be able to handle URLs such as [http://example.com/api/items/4.json][json-url].
 
 Start by adding a `format` keyword argument to both of the views, like so.
 
@@ -202,7 +202,7 @@ See the [browsable api][browsable-api] topic for more information about the brow
 
 In [tutorial part 3][tut-3], we'll start using class-based views, and see how generic views reduce the amount of code we need to write.
 
-[json-url]: http://example.com/api/items/4/.json
+[json-url]: http://example.com/api/items/4.json
 [devserver]: http://127.0.0.1:8000/snippets/
 [browsable-api]: ../topics/browsable-api.md
 [tut-1]: 1-serialization.md