pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/answering-machine/challenges.md

@@ -10,7 +10,7 @@ Complete the [answering machine](/lessons/answering-machine/activity) activity a
 basic.showString("ASK ME A QUESTION")
 ```
 
-### Challenge 1
+## Challenge 1
 
 Now we need to reply after someone asks @boardname@ a  yes or no question. We want to respond `YES` when button `A` is pressed. Add a condition for button `A` and inside it show the string `YES`.
 
@@ -23,7 +23,7 @@ input.onButtonPressed(Button.A, () => {
 
 * `Run` the code to see if it works as expected.
 
-### Challenge 2
+## Challenge 2
 
 What if @boardname@'s answer to the question is no? Let's have `NO` be displayed when button `B` is pressed. Add a condition for button `B` and inside it show the string `NO`.
 
@@ -40,6 +40,6 @@ input.onButtonPressed(Button.B, () => {
 
 * `Run` the code to see if it works as expected.
 
-### Challenge 3
+## Challenge 3
 
 When you are asked a yes or no question, do you always say yes or no? Add a condition for `on shake` that displays `MAYBE`.