FIX: Broken cite. (#7951)

Author: JulienPalard

docs/api-guide/schemas.md

@@ -421,6 +421,7 @@ If your views have related customizations that are needed frequently, you can
 create a base `AutoSchema` subclass for your project that takes additional
 `__init__()` kwargs to save subclassing `AutoSchema` for each view.
 
+[cite]: https://blog.heroku.com/archives/2014/1/8/json_schema_for_heroku_platform_api
 [openapi]: https://github.com/OAI/OpenAPI-Specification
 [openapi-specification-extensions]: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#specification-extensions
 [openapi-operation]: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#operationObject