docs: updates README.md for @opentelemetry/propagator-b3 (#2342)
docs: updates README.md for @opentelemetry/propagator-b3 (#2342)

fixes code samples provided in README.md for OpenTelemetry Propagator B3

Co-authored-by: Valentin Marchaud <1a73af9e7ae00182733b2292511b814be66f065f@vmarchaud.fr>
open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js