Improve highlighting of some code blocks. (GH-6401)

(cherry picked from commit 46936d5a71d1683dbd8ddb6d7f39aab50ecfec50)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Doc/howto/unicode.rst b/Doc/howto/unicode.rst
index 093f445..be1fefb 100644
--- a/Doc/howto/unicode.rst
+++ b/Doc/howto/unicode.rst
@@ -30,7 +30,9 @@
 
 For a while people just wrote programs that didn't display accents.
 In the mid-1980s an Apple II BASIC program written by a French speaker
-might have lines like these::
+might have lines like these:
+
+.. code-block:: basic
 
    PRINT "MISE A JOUR TERMINEE"
    PRINT "PARAMETRES ENREGISTRES"