Fix URL of package-url rust-lang repository

[Xiphoseer]

Changed from packageurl-rs to packageurl.rs because that seems to be what the actual repo/fork is using.

[iamwillbar]

@pombredanne and @stevespringett is there a preference for having a fork in the package-url org vs just linking to the upstream?

[stevespringett]

@iamwillbar it may be more difficult to govern a fork. IMO, I'd prefer the package-url org to host the authoritative/original repo, or if that's not an option, simply provide a link to another repo in the docs.

[Xiphoseer]

Superseded by #211