Update CODEOWNERS w/ correct casing on markdown files (#33919)

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## Summary of the Pull Request

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## PR Checklist

- [ ] **Closes:** #xxx
- [ ] **Communication:** I've discussed this with core contributors
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- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end user facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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.github/CODEOWNERS

@@ -12,5 +12,5 @@
 
 # locking down files that should not change
 ./LICENSE @crutkas @DHowett @ethanfangg
-./SECURITY.MD @crutkas @DHowett @ethanfangg
-./CODE_OF_CONDUCT.MD @crutkas @DHowett @ethanfangg
+./SECURITY.md @crutkas @DHowett @ethanfangg
+./CODE_OF_CONDUCT.md @crutkas @DHowett @ethanfangg