Active Storage: Explicit form field in basic example

[henrik]

There was an example of the controller code but not the view code. This also made me uncertain whether Active Storage supported to-server uploads or just direct-to-S3. Turns out to-server works.

Also fix syntax highlighting in the more advanced JS example.

[rails-bot]

r? @sgrif

(@rails-bot has picked a reviewer for you, use r? to override)

[bogdanvlviv]

Thank you for the Pull Request. LGTM.
Could you please add [ci skip] to a commit message next time when committing to Rails documentation(http://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html#contributing-to-the-rails-documentation) in order to skip tests execution.

[henrik]

Will try to remember; cheers!