Issue #26462: Doc: avoid literal_block warnings, fix syntax highlighting.

Patch by Julien Palard.
diff --git a/Doc/tutorial/controlflow.rst b/Doc/tutorial/controlflow.rst
index 2af60d0..e1ac89f 100644
--- a/Doc/tutorial/controlflow.rst
+++ b/Doc/tutorial/controlflow.rst
@@ -473,7 +473,9 @@
               client="John Cleese",
               sketch="Cheese Shop Sketch")
 
-and of course it would print::
+and of course it would print:
+
+.. code-block:: none
 
    -- Do you have any Limburger ?
    -- I'm sorry, we're all out of Limburger