Use HTMLForm::setWrapperLegendMsg() in FormSpecialPage::getForm()

There is no need to call $this->msg() directly for the fieldset legend
as there is already a method in HTMLForm to do it.

Change-Id: I771520ea230301b51cece685f47e6a606ad860be
Merge "Use HTMLForm::setWrapperLegendMsg() in FormSpecialPage::getForm()"