[3.12] Docs: use placeholders in dbm flag param docs (GH-114482) (#114497)

Also correct the default flag param for dbm.dumb.open();
it's 'c', not 'r'.
(cherry picked from commit 8c265408c51609c6b4a6788cac9cc5fea7a14888)

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