Update contributing.md with correct CLA link

Author: filmaj

.github/contributing.md

@@ -35,7 +35,7 @@ Issues labelled `good first contribution`.
 
 For your contribution to be accepted:
 
-- [x] You must have signed the [Contributor License Agreement (CLA)](https://cla-assistant.io/slackapi/bolt-python).
+- [x] You must have signed the [Contributor License Agreement (CLA)](https://cla.salesforce.com/sign-cla).
 - [x] The test suite must be complete and pass.
 - [x] The changes must be approved by code review.
 - [x] Commits should be atomic and messages must be descriptive. Related issues should be mentioned by Issue number.
@@ -57,4 +57,4 @@ If the contribution doesn't meet the above criteria, you may fail our automated
 
 ## Maintainers
 
-There are more details about processes and workflow in the [Maintainer's Guide](./maintainers_guide.md).
+There are more details about processes and workflow in the [Maintainer's Guide](./maintainers_guide.md).