pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/lucky-7/activity.md

@@ -2,11 +2,11 @@
 
 Show a number on the LED screen. 
 
-### ~avatar avatar
+## ~avatar avatar
 
 Let's learn how to show the lucky number 7 on the LED screen.
 
-### ~
+## ~
 
 We will use `show number` to display a number on the screen. The argument (`7`) is the number to display.
 
@@ -14,9 +14,9 @@ We will use `show number` to display a number on the screen. The argument (`7`)
 basic.showNumber(7)
 ```
 
-### ~avatar boothing
+## ~avatar boothing
 
 Excellent, you're ready to continue with the [challenges](/lessons/lucky-7/challenges)!
 
-### ~
+## ~
 

docs/lessons/lucky-7/challenges.md

@@ -10,7 +10,7 @@ Complete the [lucky 7 activity](/lessons/lucky-7/activity) and your code will lo
 basic.showNumber(7)
 ```
 
-### Challenge 1
+## Challenge 1
 
 But we also should pause before showing another number. Let's add a pause of 500 milliseconds.
 
@@ -19,7 +19,7 @@ basic.showNumber(7)
 basic.pause(500)
 ```
 
-### Challenge 2
+## Challenge 2
 
 What about other multiples of 7? Let's display the next multiple of 7 on the screen!
 
@@ -31,7 +31,7 @@ basic.showNumber(14)
 
 * Run the code to see if it works as expected.
 
-### Challenge 3
+## Challenge 3
 
 Keep displaying multiples of 7 such as 21 and 28...
 

docs/lessons/lucky-7/tutorial.md

@@ -4,9 +4,9 @@ Coding challenges for lucky 7.
 
 ###~ Avatar
 
-### @video td/videos/lucky-7-1-2
+## @video td/videos/lucky-7-1-2
 
-### Rebuild the game!
+## Rebuild the game!
 
 The blocks have been shuffled! Put them back together so that…
 * The blocks should be multiples of 7 and a pause between the numbers
@@ -17,7 +17,7 @@ basic.pause(500)
 basic.showNumber(14)
 ```
 
-### Hints and tips
+## Hints and tips
 
 Cut out these documentation cards to help you!