Fix code block in the HTML version of the upgrade guide [ci skip]

In 0a7ce46 I added the section about
npm package name changes to the upgrade guide. After it was merged, I
noticed an issue in the rendering of the code block in the HTML version
that doesn't appear in the corresponding markdown version. I was able to
fix it by adding an extra newline before the code block (which doesn't
affect the markdown version).

I verified the fix by running the `guides:generate:html` rake task
locally.

Before:
https://user-images.githubusercontent.com/1863540/64316451-2a774300-cfb5-11e9-985c-0697cfc23b9a.png

After:
https://user-images.githubusercontent.com/1863540/64316583-893cbc80-cfb5-11e9-9182-a2143d99f9d9.png

guides/source/upgrading_ruby_on_rails.md

@@ -117,6 +117,7 @@ to be able to rollback set
 If you were previously loading any of the `actioncable`, `activestorage`,
 or `rails-ujs` packages through npm/yarn, you must update the names of these
 dependencies before you can upgrade them to `6.0.0`:
+
 ```
 actioncable   → @rails/actioncable
 activestorage → @rails/activestorage