insert newlines into code samples in Upgrading section of Asset Pipeline doc

[alxndr]

I noticed some odd-looking code samples when reading over the Asset Pipeline doc on RailsGuides; thought I'd try to fix it up.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @kaspth (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[maclover7]

Please add [ci skip] to the commit message for documentation only changes in the future.

[alxndr]

Whoops, thanks @maclover7! (my first Rails pr...)

[maclover7]

No problem! If you'd like to, you can rebase or amend your commit and add ci skip so that way I can merge your PR faster! :)

[alxndr]

Shall do!

[maclover7]

👍 Thank you for your contribution!

[maclover7]

❤️ 💚 💙 💛 💜

[alxndr]

Glad to help!