[3.12] Fix a "doctest" block in Doc/library/turtle.rst (GH-128831) (#128836)

Fix a "doctest" block in `Doc/library/turtle.rst` (GH-128831)
(cherry picked from commit 1598e18)

Co-authored-by: Rafael Fontenelle <[email protected]>

Author: miss-islington

Doc/library/turtle.rst

@@ -979,8 +979,8 @@ Settings for measurement
       >>> turtle.heading()
       90.0
 
-      Change angle measurement unit to grad (also known as gon,
-      grade, or gradian and equals 1/100-th of the right angle.)
+      >>> # Change angle measurement unit to grad (also known as gon,
+      >>> # grade, or gradian and equals 1/100-th of the right angle.)
       >>> turtle.degrees(400.0)
       >>> turtle.heading()
       100.0