docs: minor issues with cpp protobuf example

**Description** Minor issues with https://mcap.dev/guides/cpp/protobuf - mcap::McapWriterOptions opts("protobuf") is declared but not used in in writer.open(...) - unused std::string res in fdSet(const google::protobuf::Descriptor* d) - Version: N/A - Platform: N/A **Steps To Reproduce** N/A **Expected Behavior** N/A