pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/reference/led/stop-animation.md

@@ -7,7 +7,7 @@ play.
 led.stopAnimation()
 ```
 
-### Example
+## Example
 
 This program sets up the ``stop animation`` part of the program,
 and then shows a string that you can stop with button ``B``.
@@ -19,14 +19,14 @@ input.onButtonPressed(Button.B, () => {
 basic.showString("STOP ME! STOP ME! PLEASE, WON'T SOMEBODY STOP ME?");
 ```
 
-### ~hint
+## ~hint
 
 It's important to set up ``stop animation`` before showing the
 animation, so the ``stop animation`` part of the program will be ready
 to go.
 
-### ~
+## ~
 
-### See Also
+## See Also
 
 [show animation](/reference/basic/show-animation)