docs: remove whitespaces in migration guides

[wellshs]

What does this PR address?

Thanks for good open source project!

I have been using bentoml since version 0.13.1, I checked the migration guide once I heard the news that version 1 was released this time.
While following the migration guide, I've experienced that bentofile.yaml's indent is wrong and doesn't work, so I'm trying to contribute to my favorite open source, although it's minor.

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[aarnphm]

Thanks for the fix.