[3.12] gh-112043: Align concurrent.futures.Executor.map docs with implementation (GH-114153) (#114164)

The first parameter is named 'fn', not 'func'.
(cherry picked from commit 8d26db45df479a54eccd2aced7d8a5ea9fd0ffa5)

Co-authored-by: Erlend E. Aasland <erlend@python.org>