fix content type

Author: Vladimir Kotal

apiary.apib

@@ -216,7 +216,7 @@ see https://docs.oracle.com/javase/8/docs/api/java/time/Duration.html#parse-java
 
 ### deletes messages with specified tag [DELETE]
 
-+ Request (application/text)
++ Request (text/plain)
 
         string value of the message text - only messages with the text (and specified tag) will be deleted