Typo for array indexing trait in Expressions chapter

[b-jonas0]

There's a typo in the second edition Rust Book, page Expressions, section Mutability, see "https://doc.rust-lang.org/nightly/reference/expressions.html#mutability". It currently says an expression that's an "Array indexing of a type that implements DerefMut" can give an lvalue, but in that, DerefMut is a mistake for another trait, std::ops::IndexMut.

The source code for that book page appears to be "src/expressions.md" in this repo. The following patch should fix this problem: doc-indexmut-2017-11-07-v0.diff.txt

[steveklabnik]

I think this diff looks good; do you want to open a PR with the change? That's a bit easier than a .diff :)

[bluss]

I'm unsure it's a typo.. it should cover that we can index though the usual method lookup rules, even if the outer type does not impl indexmut, it can deref (or unsize coerce?) to something that does.

[lcnr]

Fixed by #840