Document the new npm package names in the upgrade guide

[rmacklin]

Summary

As I was upgrading a rails 5.2 app to 6.0, I had to manually rename all references to the old npm packages (in the package.json dependencies and in my app's import statements) to the new package names under the @rails scope, as outlined in #34905:

actioncable   → @rails/actioncable
activestorage → @rails/activestorage
rails-ujs     → @rails/ujs

I thought it would be helpful to specifically mention this in the upgrade guide, so that's what this PR does. (I didn't include the actiontext → @rails/actiontext rename in the upgrade guide since actiontext didn't exist in rails 5.2.)