Fix a few minor typos

[brandur]

Fixes a few minor typos in the README (which were also showing up in the Rust and Crate docs):

• purproses -> purposes
• collissions -> collisions

Additionally: I was reading the docs without much context on this library on or hashing in Rust in general. The docs here mention the "FNV" algorithm as an alternative to FxHash, but as a layman reader it wasn't clear to me where FNV is used (in Rustc? as the HashMap default?). If you want to give me a one liner with some context on that, I'd be happy to flesh it out a bit and add it to the README.