[Extensions.Propagators] Remove --prereleae flag from installation instruction (#6735)

Author: Kielek

src/OpenTelemetry.Extensions.Propagators/README.md

@@ -15,7 +15,7 @@ Add a reference to the
 package in your project.
 
 ```shell
-dotnet add package --prerelease OpenTelemetry.Extensions.Propagators
+dotnet add package OpenTelemetry.Extensions.Propagators
 ```
 
 ## Configuration
@@ -26,7 +26,7 @@ Use `B3 OpenZipkin` context only:
 using OpenTelemetry;
 using OpenTelemetry.Extensions.Propagators;
 
-Sdk.SetDefaultTextMapPropagator(new B3Propagator())
+Sdk.SetDefaultTextMapPropagator(new B3Propagator());
 ```
 
 Use `B3 OpenZipkin` and `W3C Baggage` propagators at the same time: