pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/reference/basic/show-animation.md

@@ -12,12 +12,12 @@ basic.showAnimation(`
 `)
 ```
 
-### Parameters
+## Parameters
 
 * `leds` is a [string](/types/string) that shows which LEDs are on and off, in groups one after another.
 * `interval` is an optional [number](/types/number). It means the number of milliseconds to pause after each image frame.
 
-### Example: Animating a group of image frames
+## Example: Animating a group of image frames
 
 In this animation, each row is 15 spaces wide because
 there are three frames in the animation, and each frame is
@@ -33,13 +33,13 @@ basic.showAnimation(`
 `)
 ```
 
-### ~hint 
+## ~hint 
 
 If the animation is too fast, make `interval` bigger.
 
-### ~
+## ~
 
-### Example: animating frames with a pause
+## Example: animating frames with a pause
 
 This example shows six frames on the screen, pausing 500 milliseconds after each frame.
 
@@ -57,8 +57,8 @@ basic.showAnimation(`
 `, 500)
 ```
 
-### ~hint 
+## ~hint 
 
 Use [forever](/reference/basic/forever) to show an animation over and over.
 
-### ~
+## ~