Fix contribution guide link in Readme (#28023)

* Fix contribution guide link in Readme

* Fix contributing link

CONTRIBUTING.md

@@ -21,7 +21,7 @@
 
 There are many ways to contribute to Beam, just one of which is by contributing code.
 For a full list of ways to contribute and get plugged into Beam, see the
-[Beam Contribution Guide](https://beam.apache.org/contribute/get-started-contributing/)
+[Beam Contribution Guide](https://beam.apache.org/contribute/)
 
 ## Code Contributions
 
@@ -327,4 +327,4 @@ If you are contributing a ```PTransform``` to Beam, we have an extensive [PTrans
 
 If you are contributing a Runner to Beam, refer to the [Runner authoring guide](https://beam.apache.org/contribute/runner-guide/).
 
-Review [design documents](https://s.apache.org/beam-design-docs).
\ No newline at end of file
+Review [design documents](https://s.apache.org/beam-design-docs).

README.md

@@ -95,7 +95,7 @@ To get involved in Apache Beam:
 * [Report an issue](https://github.com/apache/beam/issues/new/choose).
 
 Instructions for building and testing Beam itself
-are in the [contribution guide](https://beam.apache.org/contribute/).
+are in the [contribution guide](./CONTRIBUTING.md).
 
 ## 📚 Learn More