Fix a "doctest" block in Doc/library/turtle.rst (#128831)

Author: rffontenelle

Doc/library/turtle.rst

@@ -993,8 +993,8 @@ 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