Add missing colon to IDLE doc markup (GH-22007)

diff --git a/Doc/library/idle.rst b/Doc/library/idle.rst
index 75b6fa3..43096b0 100644
--- a/Doc/library/idle.rst
+++ b/Doc/library/idle.rst
@@ -491,7 +491,7 @@
 large number of milliseconds, such as 100000000.) For imported module
 names or class or function attributes, type '.'.
 For filenames in the root directory, type :data:`os.sep` or
-data:`os.altsep` immediately after an opening quote.  (On Windows,
+:data:`os.altsep` immediately after an opening quote.  (On Windows,
 one can specify a drive first.)  Move into subdirectories by typing a
 directory name and a separator.