Fix asciidoc syntax

A code block was incorrectly labeled as java.

src/reference/asciidoc/channel.adoc

@@ -409,7 +409,7 @@ inChannel.send(new GenericMessage<String>("5"));
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 Typically this would be a perfectly legal operation, however since we are using Datatype Channel the result of such operation would generate an exception:
-[source,java]
+[source]
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 Exception in thread "main" org.springframework.integration.MessageDeliveryException:
 Channel 'numberChannel'