doc: restore color for visited links

A previous change altered the rendering of visited links. This restores
it.

PR-URL: https://github.com/nodejs/node/pull/35108
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

doc/api_assets/style.css

@@ -65,7 +65,8 @@ a.type {
 }
 
 a:link,
-a:active {
+a:active,
+a:visited {
   color: #43853d;
   text-decoration: none;
   border-radius: 2px;