Explain the placeholder import in template.py

Also add the missing quote in the instructions in README.txt.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/idlelib/idle_test/README.txt

@@ -18,7 +18,7 @@ The idle directory, idlelib, has over 60 xyz.py files. The idle_test
 subdirectory contains test_xyz.py for each implementation file xyz.py.
 To add a test for abc.py, open idle_test/template.py and immediately
 Save As test_abc.py.  Insert 'abc' on the first line, and replace
-'zzdummy' with 'abc.
+'zzdummy' with 'abc'.
 
 Remove the imports of requires and tkinter if not needed.  Otherwise,
 add to the tkinter imports as needed.

Lib/idlelib/idle_test/template.py

@@ -1,6 +1,6 @@
 "Test , coverage %."
 
-from idlelib import zzdummy
+from idlelib import zzdummy  # Replace with the module to test.
 import unittest
 from test.support import requires
 from tkinter import Tk