Installing protoc doc

https://github.com/google/proto-lens/blame/master/docs/installing-protoc.md#L32-L36

should be
```
PROTOC_ZIP=protoc-3.14.0-linux-x86_64.zip
      curl -OL https://github.com/protocolbuffers/protobuf/releases/download/v3.14.0/$PROTOC_ZIP
```