[Docs] Dead link in README

Hey there.

I noticed the link to the JSON RPC in the README is dead. It seems the file name changed in Snapcast from 
https://github.com/badaix/snapcast/blob/master/doc/json_rpc_api/v2_0_0.md
to
https://github.com/badaix/snapcast/blob/master/doc/json_rpc_api/control.md


This is not a big deal since its not hard to find, just letting you know. :)