Update a selector for help-button in about:preferences

    #help-button ->
    .help-button

Author: Masafumi Yokoyama

configuration-sheets/customization-items.ods

@@ -32,11 +32,11 @@ Firefox 31以前：
 4. globalChrome.cssで設定を反映する","@-moz-document
   url-prefix(""chrome://browser/content/preferences/in-content/preferences.xul""),
   url-prefix(""about:preferences"") {
-  #help-button {
+  .help-button {
     visibility: collapse !important;
     -moz-user-focus: ignore !important;
   }
-}",,"@-moz-document
+}",○,"@-moz-document
   url-prefix(""chrome://browser/content/preferences/in-content/preferences.xul""),
   url-prefix(""about:preferences"") {
   #help-button {