Bug 454829 followup, avoid warning about inline #text in an XUL box, r=philor

mail/base/content/multimessageview.xhtml

@@ -72,7 +72,7 @@
                      xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
                     <hbox id="buttonhbox" align="start">
                         <button id="archive" class="multimessage button"
-                                onclick="window.top.MsgArchiveSelectedMessages(null)">&archive.label;</button>
+                                onclick="window.top.MsgArchiveSelectedMessages(null)" label="&archive.label;"></button>
                         <button id="trash" class="multimessage button trashButton"
                                 onclick="window.top.DefaultController.doCommand('cmd_delete');"/>
                     </hbox>