docs: Fix broken rpc link (#4221)

- chane rpc link to master branch rpc docs, current link leads to 404
- closes #4199

Signed-off-by: Marko Baricevic <marbar3778@yahoo.com>

docs/tendermint-core/rpc.md

@@ -6,6 +6,6 @@ order: 4
 
 The RPC documentation is hosted here:
 
-- [https://tendermint.com/rpc/](https://tendermint.com/rpc/)
+- [https://docs.tendermint.com/master/rpc/](https://docs.tendermint.com/master/rpc/)
 
 To update the documentation, edit the relevant `godoc` comments in the [rpc/core directory](https://github.com/tendermint/tendermint/tree/master/rpc/core).