pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/smiley/challenges.md

@@ -25,7 +25,7 @@ basic.showLeds(`
 ```
 
 
-### Challenge 1
+## Challenge 1
 
 What if we want to make the face to frown on button pressed A?
 
@@ -52,7 +52,7 @@ input.onButtonPressed(Button.A, () => {
 })
 ```
 
-### Challenge 2
+## Challenge 2
 
 
 
@@ -87,7 +87,7 @@ input.onButtonPressed(Button.A, () => {
 
 * Run your code to see if it works as expected.
 
-### Challenge 3
+## Challenge 3
 
 When *button B* is pressed, let's change the sad face back to a happy face. To do this, begin by adding a condition for `on button pressed` *B*. Next, show LEDs as a smiley face inside the condition.