In MinHash documentation, MurmurHash3 doesn't have a `hash32` method

[davidlowryduda]

The code snippet for using MurmurHash3 doesn't work as mmh3 doesn't have a hash32 function. I suspect that these calling semantics were mirrored from Farmhash.

But there are 32-bit hash functions exposed in mmh3. I'll create a PR to fix.