Docs link is stale

[raphlinus]

The documentation line in Cargo.toml points to http://sasheldon.com/rust-objc/objc/ , which is pretty stale (it doesn't have any reference to the rc module). I think the easiest fix is to remove that line, which would make the ecosystem default to docs.rs. Happy to make that change. There might be some more metadata cleanup in order, haven't dug in deeply.