update synax for slack notification

docs/30_Usage.md

@@ -74,9 +74,7 @@ tags[]="monitoring"
 email[]="support@example.com"
 email[]="webmaster@example.com"
 
-; for slack use the following format
-; <channelname> + comma + <webhook url>
-slack[]='{ "#support-channel": "https://hooks.slack.com/services/XXXXXX/YYYYYY/ZZZZZ" }'
+slack["#support-channel"]="https://hooks.slack.com/services/XXXXXX/YYYYYY/ZZZZZ"
 
 
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