KEP-365: Fix capitalization of file suffix

[mrueg]

• All other KEPs include a README.md file, this one is called README.MD.

• Issue link:

• Other comments:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mrueg
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[k8s-ci-robot]

@mrueg: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-enhancements-verify	96e602f	link	true	/test pull-enhancements-verify

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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