Fix links for type hint changes in 2.21 blog post (apache#11947)

website/www/site/content/en/blog/beam-2.21.0.md

@@ -44,10 +44,10 @@ for example usage.
  `process()`) with `@apache_beam.typehints.no_annotations` will disable it
  for that function.
 
- More details will be in 
+ More details can be found in 
  [Ensuring Python Type Safety](https://beam.apache.org/documentation/sdks/python-type-safety/)
- and an upcoming
- [blog post](https://beam.apache.org/blog/python/typing/2020/03/06/python-typing.html).
+ and the Python SDK Typing Changes
+ [blog post](https://beam.apache.org/blog/python-typing/).
 
 * Java SDK: Introducing the concept of options in Beam Schema’s. These options add extra 
 context to fields and schemas. This replaces the current Beam metadata that is present