Fix , being used as value parser for env var configs

[gaborbernat]

Resolves #3112

[posita]

Probably shouldn't keep the :: here.

[gaborbernat]

No, the code block should handle that. :: is alternative to code-block without specifying the syntax.

[posita]

Right, but you can't have both. You can have something like this:

This is some code (note the double colon)::

    Here is my code! 😄

Or, you can have something like this:

This is some code (note the single colon):

.. code-block:: lang

    Here is my code! 😄

But this won't work (notice the double colon after the text and the use of code-block):

This is some code (note the double colon)::

.. code-block:: lang

    Where is my code?! ☹️

Proof is in the fix: #3120. (As an aside, it's stuff like this why I gave up on Sphinx/reStructuredText for my own projects. I was spending far too much time fiddling with basic things that were too easy to get wrong and should have been way more intuitive to get right. That's just an observation, not advice. Migrating probably doesn't make sense for this project, given the investment.)