Fix small typos in asset_pipeline.md

[djoume]

No description provided.

[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.

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[kaspth]

Thanks! In the future, be sure to add [ci skip] to your commit message for documentation changes, so our CI server doesn't run needlessly.