added :: and a small explanation

Author: dimpase

src/doc/en/developer/github.rst

@@ -78,11 +78,11 @@ which will show the default repo along with its readme, which is quite long.
 
 ``gh`` is extendable; e.g.  a useful extension to ``gh`` allows testing of
 Sage's GitHub Actions locally, using Docker. It is called ``act`` and can be
-installed by running
+installed by running::
 
     [alice@localhost sage]$ gh extension install https://github.com/nektos/gh-act
 
-Append ``--force`` flag to force an upgrade.
+Append ``--force`` flag to the command above to force an upgrade of the extension.
 More details on configuring and using ``gh act`` are in :ref:`chapter-portability_testing`.