KAFKA-18951: Correct topic name in client example docs

[gvdutra]

What changed

• replace stale topic1 references with the demo's actual my-topic
  topic name
• correct the README wording from "to synchronous send" to "to
  synchronously send"
• keep the Java example comments consistent with the README and
  TOPIC_NAME

Why

The producer/consumer example uses my-topic, but its documentation
still referred to topic1. This mismatch could confuse users following
the example.

Validation

• ./gradlew :examples:compileJava :examples:checkstyleMain spotlessCheck
• git diff --check

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