Fix typos
auto merge of #6345 : seanmoon/rust/fix-typos, r=sanxiyn

Hi there,

Really enjoying Rust. Noticed a few typos so I searched around for a few more--here's some fixes.

Ran `make check` and got `summary of 24 test runs: 4868 passed; 0 failed; 330 ignored`.

Thanks!

Sean