Fix broken Heroku JSON Schema link in schemas.md (#9693)

Replaced the outdated Heroku JSON Schema reference URL, which was returning a 404 error, with the updated working link from the official Heroku blog.

Author: Rohit10jr

docs/api-guide/schemas.md

@@ -451,7 +451,7 @@ 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
+[cite]: https://www.heroku.com/blog/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