Fix heading formatting in README.md

[christama]

What?

Closes

Why?

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

[github-actions]

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[github-actions]

Warning: Type of PR label mismatch

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[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @christama! In case you missed it, we'd love to have you join us in our Slack community.

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[t-hamano]

Thanks for the PR, but such a change would be meaningless.