Fix some mistakes in HRTB docs
Fix some mistakes in HRTB docs

The example code for higher-ranked trait bounds on closures had an
unnecessary `mut` which was confusing, and the text referred to an
mutable reference which does not exist in the code (and isn't needed).
Removed the `mut`s and fixed the text to better describe the actual
error for the failing example.

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