siphash link in 8.3 'Hashing Functions' 404s

[rmustacc]

I was reading through section 8.3 today and in the hashing section (https://doc.rust-lang.org/book/ch08-03-hash-maps.html#hashing-functions) there's a link to a siphash.pdf as a footnote of sorts for the cryptographically strong discussion. When I go to the link included (https://www.131002.net/siphash/siphash.pdf) I get a 404.

[steveklabnik]

Merged a fix, thanks.