[3.12] gh-120128: fix description of argument to ipaddress.collapse_addresses() (GH-120131) (#120136)

gh-120128: fix description of argument to ipaddress.collapse_addresses() (GH-120131)

The argument to collapse_addresses() is now described as an *iterable*
(rather than *iterator*).
(cherry picked from commit f878d46e5614f08a9302fcb6fc611ef49e9acf2f)

Co-authored-by: Jan Kaliszewski <zuo@kaliszewski.net>