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

Patch by Julien Palard.
diff --git a/Doc/howto/unicode.rst b/Doc/howto/unicode.rst
index afd89c7..b94e5b5 100644
--- a/Doc/howto/unicode.rst
+++ b/Doc/howto/unicode.rst
@@ -619,7 +619,9 @@
    print os.listdir('.')
    print os.listdir(u'.')
 
-will produce the following output::
+will produce the following output:
+
+.. code-block:: shell-session
 
    amk:~$ python t.py
    ['.svn', 'filename\xe4\x94\x80abc', ...]