[3.12] gh-116938: Fix `dict.update` docstring and remove erraneous full stop from `dict` documentation (GH-125421) (#126151)

gh-116938: Fix `dict.update` docstring and remove erraneous full stop from `dict` documentation (GH-125421)
(cherry picked from commit 5527c4051c0b58218ce69044f92b45f1d66ed43f)

Co-authored-by: Prometheus3375 <35541026+Prometheus3375@users.noreply.github.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>