book: Inconsistency between "a `&mut`" and "an `&mut`"

[salty-horse]

The book uses both forms several times. It should be consistent.

files with a &mut:

benchmark-tests.md
borrow-and-asref.md
guessing-game.md
references-and-borrowing.md

files with an &mut:

choosing-your-guarantees.md
choosing-your-guarantees.md
lifetimes.md
references-and-borrowing.md

[steveklabnik]

It should generally be "a", since it's "a mutable reference"