docs: fix the title overline in cli_usage/index.rst (#458)

Signed-off-by: behnazh-w <behnaz.hassanshahi@oracle.com>

Author: behnazh-w

docs/source/pages/cli_usage/index.rst

@@ -49,12 +49,13 @@ Common Options
 
 	The directory where Macaron looks for already cloned repositories.
 
------------
-Common Environment
------------
+---------------------
+Environment Variables
+---------------------
 
-``MACARON_IMAGE_TAG`` – The tag to use for the Macaron docker image
-``DOCKER_PULL`` – Whether to pull docker image from registry, must be one of: always, missing, never (default: always)
+* ``MACARON_IMAGE_TAG``: The Docker image tag for a specific version of Macaron.
+
+* ``DOCKER_PULL``: Whether to pull the Docker image from the `GitHub Container registry <https://github.com/oracle/macaron/pkgs/container/macaron>`_; must be one of: ``always``, ``missing``, ``never`` (default: ``always``).
 
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 Action Commands