Update dockerfile package name in docs

[jacobhq]

Summary of changes

Update dockerfile package name in docs (https://fly.io/docs/js/frameworks/nextjs/#generating-your-next-js-dockerfile)

Preview

Before:

After:

Related Fly.io community and GitHub links

N/A

Notes

Don't merge this yet, I'll check for other references.

[kcmartin]

Thanks for this, and sorry for the long delay! The docs follow the generator's own README: npx dockerfile is the documented post-install command (our sentence starts "Once installed..."), and the no-install form upstream recommends is npx --yes @flydotio/dockerfile@latest. This change also covers only 5 of the 7 files with the pattern, so closing. Appreciate the screenshots!