upspin/proto: update comment regarding GRPC
upspin/proto: update comment regarding GRPC

Change-Id: Ie77c340928cfc6c4864ff6a15d22ef2515e9c50e
Reviewed-on: https://upspin-review.googlesource.com/8205
Reviewed-by: Eduardo Pinheiro <c9d6fd76b920b269b838bbb70f683c6db503d8d1@gmail.com>

upspin/upspin,upspin/upspin