pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/hero/activity.md

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 A classic game, Hero, in which you must capture the food and flee away from the ghost!
 
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 This tutorial will teach you how to create a hero game to capture food while dodging the ghost; the game was inspired by the classic arcade game Pac Man.
 
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 First we need to create a function create sprite at the x, y coordinates and that set the variable called hero on the @boardname@ the first time we play.
 
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 Congratulations! You have a homemade hero game based on the classic version of PacMan
 
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