Fix API styleguide link

doc/development/api_styleguide.md

@@ -121,7 +121,7 @@ For instance:
 The [internal API](./internal_api.md) is documented for internal use. Please keep it up to date so we know what endpoints
 different components are making use of.
 
-[Entity]: https://gitlab.com/gitlab-org/gitlab/blob/master/lib/api/entities.rb
+[Entity]: https://gitlab.com/gitlab-org/gitlab/blob/master/lib/api/entities
 [validation, and coercion of the parameters]: https://github.com/ruby-grape/grape#parameter-validation-and-coercion
 [installing GitLab under a relative URL]: https://docs.gitlab.com/ee/install/relative_url.html