docs: fix security issue notifications link in readme (#245)

* docs: fix security issue notifications link in readme

* docs: update README.rst

Co-Authored-By: seebees <ryanemer@amazon.com>

Co-authored-by: seebees <ryanemer@amazon.com>

Author: mattsb42-aws

README.rst

@@ -117,4 +117,4 @@ to your use-case in order to obtain peak performance.
 .. _final message: https://docs.aws.amazon.com/encryption-sdk/latest/developer-guide/message-format.html
 .. _encryption context: https://docs.aws.amazon.com/kms/latest/developerguide/concepts.html#encrypt_context
 .. _examples: https://github.com/aws/aws-encryption-sdk-python/tree/master/examples
-.. _Security issue notifications: ./CONTRIBUTING.md#security-issue-notifications
+.. _Security issue notifications: https://github.com/aws/aws-encryption-sdk-python/tree/master/CONTRIBUTING.md#security-issue-notifications