rustbook: Decrease the max-width for the mobile view

This should fix #22682.

"max-width: 1023px" seems like a good compromise here. This way,
1024x768 monitors will still see the sidebar (plus, iPad in landscape
mode).

src/rustbook/static/rustbook.css

@@ -55,7 +55,7 @@ h1, h2, h3, h4, h5, h6 {
     }
 }
 
-@media only screen and (max-width: 1060px) {
+@media only screen and (max-width: 1023px) {
     #toc {
         width: 100%;
         top: 40px;