Update documentation for "kafka" extension

[shpakkrll]

Describe the bug
According to documentation https://quarkus.io/guides/kafka-streams while adding extension via
./mvnw quarkus:add-extension -Dextensions="kafka"
This should add the following to your pom.xml:

<dependency>
    <groupId>io.quarkus</groupId>
    <artifactId>quarkus-smallrye-reactive-messaging-kafka</artifactId>
</dependency>

In practice it's failed because there is:
❌ Multiple extensions matching 'kafka'

In a scenario where the user trying to add multiple extensions at once, it takes some time to find the extension that causes the problem (because it's not mentioned on which one ./mvnw quarkus:add-extension fails)

Resolution
Update listed command in documentation with ./mvnw quarkus:add-extension -Dextensions="quarkus-smallrye-reactive-messaging-kafka"

Screenshots

Environment (please complete the following information):
mvn io.quarkus:quarkus-maven-plugin:1.8.1.Final