pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/lessons/strobe-light/activity.md

@@ -2,13 +2,13 @@
 
 Develop shapes with a for loop. 
 
-### ~avatar avatar
+## ~avatar avatar
 
 
 
 Welcome! This guided tutorial will teach how to develop shapes with a for loop. Let's get started!
 
-### ~
+## ~
 
 Create a `for loop` that will loop from 0 to 4 to indicate the x-coordinates of the @boardname@ display.
 
@@ -54,9 +54,9 @@ for (let i = 0; i < 5; i++) {
 }
 ```
 
-### ~avatar avatar
+## ~avatar avatar
 
 Excellent, you're ready to continue with the [challenges](/lessons/strobe-light/challenges)!
 
-### ~
+## ~
 

docs/lessons/strobe-light/challenges.md

@@ -17,7 +17,7 @@ for (let i = 0; i < 5; i++) {
 ```
 
 
-### Challenge 1
+## Challenge 1
 
 
 
@@ -34,7 +34,7 @@ for (let i = 0; i < 5; i++) {
 
 * Run the code to see if it works as expected.
 
-### Challenge 2
+## Challenge 2
 
 
 
@@ -51,7 +51,7 @@ for (let i = 0; i < 5; i++) {
 
 * Run the code to see if it works as expected.
 
-### Challenge 3
+## Challenge 3