[ruby/rdoc] Use pointer cursor for navigation toggle

(https://github.com/ruby/rdoc/pull/1175)

https://github.com/ruby/rdoc/commit/964a1982c8

lib/rdoc/generator/template/darkfish/css/rdoc.css

@@ -223,6 +223,7 @@ nav a:hover {
   position: fixed;
   top: 10px;
   left: 20px;
+  cursor: pointer;
 }
 
 #navigation-toggle[aria-expanded="true"] {