Fix gRPC API link in README
* [Docs](https://matchbox.psdn.io/)
* [Configuration](docs/config.md)
-* [HTTP API](docs/api-http.md) / [gRPC API](docs/grpc-api.md)
+* [HTTP API](docs/api-http.md) / [gRPC API](docs/api-grpc.md)
## Installation
