Example contained unnecessary explicit type info for the vector in Mutex v. Rust will magically do the needful conversions for us. Code looks cleaner/simpler without the explicit typing.
* Update mutex.md
Adding speaker notes why Rust `Mutex` has its design and mentioning briefly `RwLock`.
Someone from the audience can notice `unwrap()` in the code, it might be worth to have the answer in speaker notes.
* Apply suggestions from code review
Co-authored-by: Martin Geisler <martin@geisler.net>