Close issue #16077: fix code example in documentation of reduce() built-in (from docs@).

Doc/library/functions.rst

@@ -1100,7 +1100,7 @@ available.  They are listed here in alphabetical order.
               except StopIteration:
                   raise TypeError('reduce() of empty sequence with no initial value')
           accum_value = initializer
-          for x in iterable:
+          for x in it:
               accum_value = function(accum_value, x)
           return accum_value