minor: Update mongocxx release instructions
minor: Update mongocxx release instructions

acmorrow/mongo-cxx-driver,xdg/mongo-cxx-driver,mongodb/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,xdg/mongo-cxx-driver,xdg/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,xdg/mongo-cxx-driver