Clarify in the book that indexing into a vector must be done with a usize

[bstrie]

Someone came into the channel today confused at how to index into a vector. Their problem was that the variable that they wanted to use as an index was an i8. Casting actually crossed their mind, but they gave up after trying v[foo as i32] and v[foo as isize]. The page at http://doc.rust-lang.org/stable/book/vectors.html could stand to have a sentence mentioning how to index with a variable (currently it just indexes with constants, which all properly get inferred to usize without the reader realizing).