doc: reduce font size on smaller screens

The font size for tablet sized screens (or half a laptop screen) is
overly large, reduce it to a more readable size.

Fixes: nodejs/nodejs.org#919

PR-URL: nodejs/node#11695
Fixes: https://github.com/nodejs/nodejs.org#919
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

doc/api_assets/style.css

@@ -476,7 +476,7 @@ th > *:last-child, td > *:last-child {
 
 @media only screen and (max-width: 1024px) {
   #content {
-    font-size: 2.5em;
+    font-size: 1.6em;
     overflow: visible;
   }
   #column1.interior {