IDLE: Fix typos in docs and comments (GH-13749)

diff --git a/Lib/idlelib/config.py b/Lib/idlelib/config.py
index aa94d65..12113c1 100644
--- a/Lib/idlelib/config.py
+++ b/Lib/idlelib/config.py
@@ -12,7 +12,7 @@
 there are  separate dicts for default and user values.  Each has
 config-type keys 'main', 'extensions', 'highlight', and 'keys'.  The
 value for each key is a ConfigParser instance that maps section and item
-to values.  For 'main' and 'extenstons', user values override
+to values.  For 'main' and 'extensions', user values override
 default values.  For 'highlight' and 'keys', user sections augment the
 default sections (and must, therefore, have distinct names).