Grammar fix in tutorial (GH-18425)

Author: donkirkby

Doc/tutorial/controlflow.rst

@@ -142,7 +142,7 @@ the list, thus saving space.
 We say such an object is :term:`iterable`, that is, suitable as a target for
 functions and constructs that expect something from which they can
 obtain successive items until the supply is exhausted.  We have seen that
-the :keyword:`for` statement is such a construct, while an example of function
+the :keyword:`for` statement is such a construct, while an example of a function
 that takes an iterable is :func:`sum`::
 
     >>> sum(range(4))  # 0 + 1 + 2 + 3