Fix unclear make_mut docs
Fix unclear make_mut docs

Changed the description of the `make_mut` copy-on-write behaviour in arc.rs

The sentence "doesn't have one strong reference and no weak references." is a
hard to understand double negative, which can be much more easily explained.

GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust