readme: point arvo URL at subtree [ci skip]
This one was still pointing at the deprecated arvo repository.
@@ -11,7 +11,7 @@ A personal server operating function.
[azim]: https://etherscan.io/address/0x223c067f8cf28ae173ee5cafea60ca44c335fecb
[aens]: https://etherscan.io/address/azimuth.eth
[brid]: https://github.com/urbit/bridge/releases
-[arvo]: https://github.com/urbit/arvo/
+[arvo]: https://github.com/urbit/urbit/tree/master/pkg/arvo
## Install
