Fix protoc command in README

Executing protoc from top level folder resulted in 
```
Asset.proto: File not found.
Download.proto: File not found.
proto/App.proto: Import "Asset.proto" was not found or had errors.
proto/App.proto: Import "Download.proto" was not found or had errors.
proto/App.proto:33:18: "FileMetadata" is not defined.
proto/App.proto:38:18: "ExternalAssetProto" is not defined.
```

Thus I had to change to the proto dir before compiling.