Update protobuf readme

- Add required version for grpc tools. Newer versions cause UnityToExternalGrpc.cs to fail to compile inside Unity due to a new function in the file

protobuf-definitions/README.md

 If you don't have it already, download the latest version of [nuget](https://www.nuget.org/downloads).
 Navigate to your installation of nuget and run the following: 
 
-`nuget install Grpc.Tools -OutputDirectory $MLAGENTS_ROOT\protobuf-definitions`
+`nuget install Grpc.Tools -Version 1.14.1 -OutputDirectory $MLAGENTS_ROOT\protobuf-definitions`
 
 ### Installing Protobuf Compiler