Change the broken reference.

Author: Oleksii Kliukin

CONTRIBUTING.md

@@ -4,7 +4,7 @@ Wanna contribute to the Postgres Operator? Yay - here is how!
 
 ## Reporting issues
 
-If you have a question about patroni or have a problem using it, please read the :ref:`README <readme>` before filing an issue.
+If you have a question about patroni or have a problem using it, please read the`README` before filing an issue.
 Also double check with the current issues on our [Issues Tracker](https://github.com/zalando-incubator/postgres-operator/issues).
 
 ## Contributing a pull request