Bug 1620078 - DevTools docs - fix title overline (lint failure fix). r=Honza

Depends on D67179

Differential Revision: https://phabricator.services.mozilla.com/D69464

--HG--
extra : moz-landing-system : lando

devtools/docs/index.rst

@@ -1,6 +1,6 @@
-========
+=================================
 Firefox DevTools Contributor Docs
-========
+=================================
 
 This is a guide to working on the code for Firefox Developer Tools. If you're looking for help with using the tools, see the `user docs <https://developer.mozilla.org/en-US/docs/Tools>`__. For other ways to get involved, check out our `community site <https://firefox-dev.tools/>`__.