Syntax highlight IDLE html doc code example. (GH-12981)


The new markup is currently ignored by IDLE's tk doc display.
(cherry picked from commit 55d035113dfb1bd90495c8571758f504ae8d4802)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
diff --git a/Doc/library/idle.rst b/Doc/library/idle.rst
index 11e137b..ad44911 100644
--- a/Doc/library/idle.rst
+++ b/Doc/library/idle.rst
@@ -724,11 +724,9 @@
 characters cause following text to appear on a new line.  Other control
 characters are ignored or displayed as a space, box, or something else,
 depending on the operating system and font.  (Moving the text cursor through
-such output with arrow keys may exhibit some surprising spacing behavior.)
+such output with arrow keys may exhibit some surprising spacing behavior.) ::
 
-.. code-block:: none
-
-   >>> s = 'a\tb\a<\x02><\r>\bc\nd'
+   >>> s = 'a\tb\a<\x02><\r>\bc\nd'  # Enter 22 chars.
    >>> len(s)
    14
    >>> s  # Display repr(s)