Small fix for the router visualizer

[bruno-]

This is a small fix for the router visualizer. The problem is that the routes overlap with the image.

Here's the screenshot how this looks now (for a dummy/scaffolded app):

Screenshot for an app with a lot of routes:

This is how it looks when fixed (the routes are just rendered below the image):

For completeness sake, here's how to get the routes visualizer:

# in project root
bundle exec rails r 'File.write("./viz.html", Rails.application.routes.router.visualizer)'
open ./viz.html

Edit: btw. this was checked on chrome and safari.