Propagators API minor clarifications (#315)
Propagators API minor clarifications (#315)

* Propagators API minor clarifications

Clarifies that `Setter` and `Getter` are arguments on `Extract` and `Inject` and not separate members of `HttpTextFormat`

* Update api-propagators.md

open-telemetry/opentelemetry-specification,open-telemetry/opentelemetry-specification