UnitControl: Fixed documentation for units attribute

[hsmio]

Description

This PR fixes typos and omissions in documentation and Storybook to properly document the property units of the UnitControl component.

Problem solved: When looking at storybook & docs, it was unclear if/how we could define the units available in the component <UnitControl/>.

How has this been tested?

I created a demo here to make sure the syntax is correct: https://codesandbox.io/s/beautiful-burnell-3d4gm?file=/src/App.js and ran npm run storybook:dev to test the Storybook result (see screenshots).

Screenshots

See it in action: https://user-images.githubusercontent.com/5040227/118523628-3711df80-b735-11eb-9623-3d698a7137f9.mp4

Types of changes

Documentation fixes (non-breaking)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @hsmio! In case you missed it, we'd love to have you join us in our Slack community, where we hold regularly weekly meetings open to anyone to coordinate with each other.

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[jorgefilipecosta]

LGTM 👍 Thank you for this enhancement!

[github-actions]

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