pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/answering-machine/activity.md

@@ -2,12 +2,12 @@
 
 Learn to create an answering machine on the @boardname@ 
 
-### ~avatar avatar
+## ~avatar avatar
 
 
 Let's learn how to create an answering machine!
 
-### ~
+## ~
 
 We will use `show string` to show text on the LED screen. *String* is a common name for *text* in programming languages. The function `show string` scrolls the text column by column at a *150* milliseconds interval.
 
@@ -15,9 +15,9 @@ We will use `show string` to show text on the LED screen. *String* is a common n
 basic.showString("ASK ME A QUESTION")
 ```
 
-### ~avatar boothing
+## ~avatar boothing
 
 Excellent, you're ready to continue with the [challenges](/lessons/answering-machine/challenges)!
 
-### ~
+## ~
 

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`.