Add "m" as valid memory resource unit in docs

[jcook793]

This is a Bug Report

Problem: The docs for memory resource units do not list "m" as being a valid unit, but then immediately use it in an example.

Proposed Solution: Add "m" as a valid memory unit (millibytes), but keep the example and warning about case sensitivity.

Page to Update:
https://kubernetes.io/docs/concepts/configuration/manage-resources-containers/#meaning-of-memory

[k8s-ci-robot]

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