doc: use valid language in code-blocks

See ¹ for the valid values.

¹: https://www.sphinx-doc.org/en/1.5/markup/code.html#directive-code-block

poc/README.rst

@@ -12,8 +12,8 @@ Try the software within this directory with caution, anything can break
 at any moment.
 
 
-.. code-block:: plain
-   
+.. code-block:: none
+
             _,-._
            ; ___ :           ,------------------------------.
        ,--' (. .) '--.__    |                                |