[Docs] Dead link in README

[botondf]

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. :)

[beanscg]

Verified this against current develop:

• Source: README.md:7 still points the Snapcast JSON-RPC API link at https://github.com/badaix/snapcast/blob/master/doc/json_rpc_api/v2_0_0.md.
• Current behavior: that URL redirects to the current snapcast/snapcast repo but the old v2_0_0.md path returns HTTP 404.
• Current API docs candidate: https://github.com/snapcast/snapcast/blob/master/doc/json_rpc_api/control.md returns HTTP 200.
• Duplicate check: the open PRs I saw are unrelated app feature/link changes and do not appear to cover this JSON-RPC docs link.

Smallest fix looks like updating the README JSON-RPC API link to the current control.md documentation path.