fixed mistaken error message
fixed mistaken error message

r-arias/mongo-rust-driver-prototype,DenisKolodin/mongo-rust-driver-prototype,saghm/mongo-rust-driver-prototype,kyeah/mongo-rust-driver-prototype,mongodb-labs/mongo-rust-driver-prototype,saghm/mongo-rust-driver-prototype,mongodbinc-interns/mongo-rust-driver-prototype,mongodb-labs/mongo-rust-driver-prototype