Fixed DCO link

Author: emlaver

CONTRIBUTING.rst

@@ -8,7 +8,7 @@ Developer Certificate of Origin
 ===============================
 
 In order for us to accept pull-requests, the contributor must sign-off a
-[Developer Certificate of Origin (DCO)](DCO1.1.txt). This clarifies the
+`Developer Certificate of Origin (DCO) <DCO1.1.txt>`_. This clarifies the
 intellectual property license granted with any contribution. It is for your
 protection as a Contributor as well as the protection of IBM and its customers;
 it does not change your rights to use your own Contributions for any other