chore: Fix anchor in README.rst.

Sphinx linkcheck seems to error if the anchor doesn't exactly match the id= value, even if the anchor itself works.

Author: lizroth

README.rst

@@ -195,4 +195,4 @@ of the one that the client would normally construct for you.
 .. _CryptoConfig: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/encrypted/config.html
 .. _decrypt_dynamodb_item: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/encrypted/item.html#dynamodb_encryption_sdk.encrypted.item.decrypt_dynamodb_item
 .. _transformation functions: https://aws-dynamodb-encryption-python.readthedocs.io/en/latest/lib/tools/transform.html
-.. _Security issue notifications: https://github.com/aws/aws-dynamodb-encryption-python/tree/master/CONTRIBUTING.md#security-issue-notifications
+.. _Security issue notifications: https://github.com/aws/aws-dynamodb-encryption-python/blob/master/CONTRIBUTING.md#user-content-security-issue-notifications