docs: updated contributing.md hyperlink (#2296)

Co-authored-by: Davis Raymond <[email protected]>

README.rst

@@ -52,7 +52,7 @@ Contributing
 ------------
 
 If you would like to contribute code please read
-`Contributing Code to Ona Data <https://github.com/onaio/onadata/wiki/Contributing-Code-to-OnaData>`_.
+`Contributing Code to Ona Data <CONTRIBUTING.MD>`_.
 
 Edit top level requirements in the file `requirements/base.in <requirements/base.in>`_. Use
  `pip-compile <https://github.com/nvie/pip-tools>`_ to update `requirements/base.pip <requirements/base.pip>`_.