Fixes #6889 doc fix [skip ci]

framework/web/Response.php

@@ -87,7 +87,7 @@ class Response extends \yii\base\Response
      * - [[FORMAT_RAW]]: the data will be treated as the response content without any conversion.
      *   No extra HTTP header will be added.
      * - [[FORMAT_HTML]]: the data will be treated as the response content without any conversion.
-     *   The "Content-Type" header will set as "text/html" if it is not set previously.
+     *   The "Content-Type" header will set as "text/html".
      * - [[FORMAT_JSON]]: the data will be converted into JSON format, and the "Content-Type"
      *   header will be set as "application/json".
      * - [[FORMAT_JSONP]]: the data will be converted into JSONP format, and the "Content-Type"