[3.12] Fix docstring and var name of itertools recipe (GH-112113) (#112310)

Fix docstring and var name of itertools recipe (GH-112113)

`prepend()` works with arbitrary iterables, not only iterators. In fact,
the example given uses a `list`, which is iterable, but not an iterator.
(cherry picked from commit 6c47eaccfa2550c140a24bc6e520d968731d9689)

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>