[Form] tweaked a deprecation message

Author: fabpot

Form.php

@@ -848,7 +848,7 @@ public function getErrors($deep = false, $flatten = true)
      */
     public function getErrorsAsString($level = 0)
     {
-        trigger_error('The '.__METHOD__.' method is deprecated since version 2.5 and will be removed in 3.0. Use the Form::getErrors(true, false) method instead and cast the result to a string.', E_USER_DEPRECATED);
+        trigger_error('The '.__METHOD__.' method is deprecated since version 2.5 and will be removed in 3.0. Use (string) Form::getErrors(true, false) instead.', E_USER_DEPRECATED);
 
         return self::indent((string) $this->getErrors(true, false), $level);
     }