protoc-gen-go-grpc: confusion between --go-grpc_out and --go-grpc_opt in README

[ggratte]

--go-grpc_out=require_unimplemented_servers
should be
--go-grpc_opt=require_unimplemented_servers
https://github.com/grpc/grpc-go/blob/master/cmd/protoc-gen-go-grpc/README.md

[dfawley]

--go-grpc_out is correct if you use :. at the end as documented.

So the documentation is correct. But maybe we should change this anyway to use --go-grpc_out=. --go-grpc_opt=[etc] because it isn't the first time people have reported being confused by it.