Fix wrong method name

This is code block for explaining methods to set up producer and consumer properties. So 'consumerProps()' and 'producerProps()' are must be here. But there is 'senderProps()' method instead of 'producerProps().

API ref. `org.springframework.kafka.test.utils.KafkaTestUtils#producerProps`

src/reference/asciidoc/testing.adoc

@@ -26,7 +26,7 @@ public static Map<String, Object> consumerProps(String group, String autoCommit,
  * @param embeddedKafka a {@link EmbeddedKafkaBroker} instance.
  * @return the properties.
  */
-public static Map<String, Object> senderProps(EmbeddedKafkaBroker embeddedKafka) { ... }
+public static Map<String, Object> producerProps(EmbeddedKafkaBroker embeddedKafka) { ... }
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