[3.9] Fix typo in turtledemo.two_canvases. (GH-25194)

(cherry picked from commit 4e2ef70)
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Lib/turtledemo/two_canvases.py

@@ -1,7 +1,7 @@
 """turtledemo.two_canvases
 
 Use TurtleScreen and RawTurtle to draw on two
-distinct canvases in a separate windows. The
+distinct canvases in a separate window. The
 new window must be separately closed in
 addition to pressing the STOP button.
 """