remove incorrect information about the api version

The API version is currently not equal to the gitlab major version number. Gitlab 4.1 
still uses API version 3. Point to the lib/api.rb file instead which contains the
autoritative information.

doc/api/README.md

@@ -10,7 +10,7 @@ If no, or an invalid, `private_token` is provided then an error message will be
 }
 ```
-API requests should be prefixed with `api` and the API version. The API version is equal to the GitLab major version number, which is defined in `lib/api.rb`.
+API requests should be prefixed with `api` and the API version. The API version is defined in `lib/api.rb`.
 Example of a valid API request: