pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/reference/images/create-big-image.md

@@ -14,13 +14,13 @@ images.createBigImage(`
     `);
 ```
 
-### Parameters
+## Parameters
 
 * ``leds`` is a [string](/types/string) that says which LEDs
 on the screen should be on and which should be off.
 
 
-### Example: Flip-flopping arrow
+## Example: Flip-flopping arrow
 
 This program makes a big image with a picture of an arrow pointing up,
 and an arrow pointing down.  If you press button `A`, the program will
@@ -44,7 +44,7 @@ input.onButtonPressed(Button.B, () => {
 });
 ```
 
-### See also
+## See also
 
 [image](/reference/images/image),
 [create image](/reference/images/create-image),