pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/digi-yoyo/activity.md

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 Create a counter with a while loop.
 
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 Welcome! This tutorial will teach how to create a counter with a while loop. Let's get started!
 
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 Create a variable that acts as a counter and set it to 0.
 
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 Excellent, you're ready to continue with the [challenges](/lessons/digi-yoyo/challenges)!
 
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