pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/beautiful-image/activity.md

@@ -2,11 +2,11 @@
 
 Generate and show a beautiful image.
 
-### ~avatar avatar
+## ~avatar avatar
 
 Let's learn how to show an image on the LED screen.
 
-### ~
+## ~
 
 We will use *show LEDs* to draw an image on the LED screen. This function immediately writes on the screen.
 
@@ -20,9 +20,9 @@ basic.showLeds(`
     `)
 ```
 
-### ~avatar boothing
+## ~avatar boothing
 
 Excellent, you're ready to continue with the [challenges](/lessons/beautiful-image/challenges)!
 
-### ~
+## ~
 

docs/lessons/beautiful-image/challenges.md

@@ -16,7 +16,7 @@ basic.showLeds(`
     `)
 ```
 
-### Challenge 1
+## Challenge 1
 
 Now show an new image that will display on the @boardname@.
 
@@ -39,7 +39,7 @@ basic.showLeds(`
 
 * Does your code work as expected?
 
-### Challenge 2
+## Challenge 2
 
 Nice job! Why don't we create a third image that will show after the other two?
 

docs/lessons/beautiful-image/tutorial.md

@@ -2,9 +2,9 @@
 
 Beautiful Image tutorial. 
 
-### ~avatar avatar
+## ~avatar avatar
 
-### @video td/videos/beautiful-image-0
+## @video td/videos/beautiful-image-0
 
 Rebuild the game!