pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/reference/images/create-image.md

@@ -13,12 +13,12 @@ images.createImage(`
 `)
 ```
 
-### 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
 
 If you press button `A`, this program will make a picture of an
 arrow and show it on the LED screen. If you press button `B`, the
@@ -45,7 +45,7 @@ input.onButtonPressed(Button.B, () => {
 });
 ```
 
-### See also
+## See also
 
 [image](/reference/images/image),
 [create big image](/reference/images/create-big-image),