rust/docs: fix critical typo for missing_docs lint

Fix typo from fe66d06a.
The exclamation point is what lets an attribute apply to
an entire crate, without the ! it's practically a placebo.

Fix on commits af182d4a and
b6059297, and note there are
still missing docs in both crypto and protover, for now.

https://doc.rust-lang.org/reference/attributes.html