pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/screen-wipe/activity.md

@@ -2,13 +2,13 @@
 
 Clear the screen by pressing buttons on the @boardname@ 
 
-### ~avatar avatar
+## ~avatar avatar
 
 
 
 This activity will teach how to clear the screen by pressing button A on the @boardname@.
 
-### ~
+## ~
 
 You can use the clear screen` function to turn off all the LED on the screen. Let's illustrate this concept with a small script where the user has to press the button A to turn off the screen. Let's start by adding the code to show an animation.
 
@@ -82,9 +82,9 @@ input.onButtonPressed(Button.A, () => {
 
 *Run* the script in the simulator or on the @boardname@ to see how this works!
 
-### ~avatar boothing
+## ~avatar boothing
 
 Excellent, you're ready to continue with the [challenges](/lessons/screen-wipe/challenges)!
 
-### ~
+## ~
 

docs/lessons/screen-wipe/challenges.md

@@ -41,7 +41,7 @@ input.onButtonPressed(Button.A, () => {
 
 ```
 
-### Challenge 1
+## Challenge 1
 
 Create an event handler for Button B.
 
@@ -83,7 +83,7 @@ input.onButtonPressed(Button.B, () => {
 
 ```
 
-### Challenge 2
+## Challenge 2
 
 
 
@@ -155,7 +155,7 @@ basic.showLeds(`
 ```
 
 
-### Challenge 3
+## Challenge 3
 
 Show an animation that scrolls back up when you press button "B".