Docs: Clarify the before_and_after() example (GH-28458) (#28464)

(cherry picked from commit fcbf9b176b1190301c760a921601c6488ef8b070)

Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>

Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>
diff --git a/Lib/test/test_itertools.py b/Lib/test/test_itertools.py
index aa81076..a12f6f0 100644
--- a/Lib/test/test_itertools.py
+++ b/Lib/test/test_itertools.py
@@ -2605,10 +2605,11 @@ def test_permutations_sizeof(self):
 >>> list(odds)
 [1, 3, 5, 7, 9]
 
->>> all_upper, remainder = before_and_after(str.isupper, 'ABCdEfGhI')
->>> str.join('', all_upper)
+>>> it = iter('ABCdEfGhI')
+>>> all_upper, remainder = before_and_after(str.isupper, it)
+>>> ''.join(all_upper)
 'ABC'
->>> str.join('', remainder)
+>>> ''.join(remainder)
 'dEfGhI'
 
 >>> list(powerset([1,2,3]))