Bug 668192 - Improve screen reader reading of password manager intro text, r=gavin

toolkit/components/passwordmgr/content/passwordManager.xul

@@ -84,7 +84,7 @@
                oncommand="_filterPasswords();"/>
     </hbox>
 
-    <description control="signonsTree" id="signonsIntro"/>
+    <label control="signonsTree" id="signonsIntro"/>
     <separator class="thin"/>
     <tree id="signonsTree" flex="1" style="height: 20em;" hidecolumnpicker="true"
           onkeypress="HandleSignonKeyPress(event)"