Update and rename collections.md to collections. MD

[Apetree100122]

Contributor guide pull request

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[Apetree100122]

#sign-off

[carlyrevier]

@Apetree100122 - Thank you for being a contributor. I'm closing this PR because it doesn't appear to change anything about this article except to change the file extension to an illegal extension. If this was not the change you intended to make, please feel free to try again!