Add a link to whitelist raw file in lastgenre documentation

it makes it easier to copy for someone wanting to create a custom whitelist, but not from scratch.

An alternative is to mention the curl command to use as proposed by @sampsyo 

> We might be better off just documenting 
>     curl -O https://raw.githubusercontent.com/sampsyo/beets/master/beetsplug/lastgenre/genres.txt 
> as the canonical way to obtain/edit the whitelist. 

but I thought a discreet link was sufficient.