SECURITY: Use message label instead of sanitized text output for massmessage-form-page-help message

* Follow-up bug fix to the released security patch:
  Ife6fb590af53fa0d8eb59201ce88a3c47ddde45c

Bug: T347742
Change-Id: I8a45d0d022e6e083d60423234443329d8951b4d4
(cherry picked from commit 432912514b)

includes/Specials/SpecialMassMessage.php

@@ -181,7 +181,7 @@ class SpecialMassMessage extends FormSpecialPage {
 			'tabindex' => $controlTabIndex++,
 			'label-message' => 'massmessage-form-page',
 			'default' => $request->getText( 'page-message' ),
-			'help-message' => $this->msg( 'massmessage-form-page-help' )->text(),
+			'help-message' => 'massmessage-form-page-help',
 			'required' => false
 		];