stable32: Fix incorrect heading markup in example_centos.rst

[brlin-tw]

☑️ Resolves

Fixes #13633 for stable32.

🖼️ Screenshots

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[brlin-tw]

@joshtrichards

Thanks.l! For future reference, you should be able to just make the change once (against the master branch). Once that is merged you/any of us can trigger the backport port for relevant older branches to generate PRs automatically with the change. ;-)

Gotcha. Thanks for the heads-up!