typo in icon file address

was way too quick on my copy-pasta

content/netError.xhtml

@@ -31,7 +31,7 @@
         &resurrect.archive;
       </button>
       <button value='archiveis'>
-        <img src='chrome://resurrect/skin/cacheicons/archiveis.ico' />
+        <img src='chrome://resurrect/skin/cacheicons/archiveis.png' />
         &resurrect.archiveis;
       </button>
       <button value='webcitation'>