[3.7] Fix parameter names in assertIn() docs (GH-18829) by miss-islington · Pull Request #19564 · python/cpython

The names "member" and "container" for the arguments are also used in the module and shown with the help() function, and are immediately understandable in this context, contrary to "first" and "second".
(cherry picked from commit a388bbd)

Co-authored-by: Christoph Zwerschke <cito@online.de>