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 <contact@vmarchaud.fr>

Author: OmkarKirpan

packages/opentelemetry-propagator-b3/README.md

@@ -86,7 +86,7 @@ api.propagation.setGlobalPropagator(new B3Propagator());
 
 ```javascript
 const api = require('@opentelemetry/api');
-const { B3Propagator } = require('@opentelemetry/propagator-b3');
+const { B3Propagator, B3InjectEncoding } = require('@opentelemetry/propagator-b3');
 
 api.propagation.setGlobalPropagator(
   new B3Propagator({ injectEncoding: B3InjectEncoding.MULTI_HEADER })
@@ -101,7 +101,8 @@ propagator.
 
 ```javascript
 const api = require('@opentelemetry/api');
-const { B3Propagator } = require('@opentelemetry/propagator-b3');
+const { CompositePropagator } = require('@opentelemetry/core');
+const { B3Propagator, B3InjectEncoding } = require('@opentelemetry/propagator-b3');
 api.propagation.setGlobalPropagator(
   new CompositePropagator({
     propagators: [