pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/glowing-pendulum/activity.md

@@ -100,9 +100,9 @@ basic.forever(() => {
 });
 ```
 
-### ~avatar avatar
+## ~avatar avatar
 
 Excellent, you're ready to continue with the [challenges](/lessons/glowing-pendulum/challenges)!
 
-### ~
+## ~
 

docs/lessons/glowing-pendulum/challenges.md

@@ -24,13 +24,13 @@ basic.forever(() => {
 
 ```
 
-### Challenge 1
+## Challenge 1
 
 ![](/static/mb/lessons/glowing-pendulum-0.jpg)
 
 Hold the @boardname@ in your hand in a dark room. Move the @boardname@ like a pendulum and produce a slow image that captures the pattern of the @boardname@ LEDs.
 
-### Challenge 2
+## Challenge 2
 
 Replace "y" in `acceleration(y)` with "x" or "z". Changing the axis will cause the @boardname@ to measure the force in a different direction. What differences in the resulting pattern does this replacement make?
 

docs/lessons/glowing-pendulum/tutorial.md

@@ -2,7 +2,7 @@
 
 The glowing pendulum changes the screen brightness based on the acceleration measured on the @boardname@.
 
-### Rebuild the game!
+## Rebuild the game!
 
 The blocks have been shuffled! Put them back together so that...
 * all LEDs are turned on