Update README.md

Author: 4n4nd

README.md

@@ -31,7 +31,7 @@ To install directly from this branch:
 
 - [Slack](https://join.slack.com/share/zt-kw3v8t1e-hbcVH7X7bXORiQuQtsNZ4A)
 - [Google Chat](https://chat.google.com/room/AAAAzFPwq5s)
-- [Documentation](https://prometheus-api-client-python.readthedocs.io/en/master/source/prometheus_api_client.html)
+- [Documentation](https://prometheus-api-client-python.readthedocs.io/en/latest/source/prometheus_api_client.html)
 
 ## Getting Started