[ruby/rdoc] Allow method source code to scroll [ci skip]

https://github.com/ruby/rdoc/commit/1bb0496c53

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

@@ -505,7 +505,7 @@ main header h3 {
 
 main .method-source-code {
   max-height: 0;
-  overflow: hidden;
+  overflow: auto;
   transition-duration: 200ms;
   transition-delay: 0ms;
   transition-property: all;