pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/reference/basic/show-leds.md

@@ -13,13 +13,13 @@ basic.showLeds(`
 )
 ```
 
-### Parameters
+## Parameters
 
 * `leds` is a [string](/types/string) that controls which LEDs are on and off.
 * `interval` is an optional [number](/types/number) that means how many milliseconds to wait after showing a picture.
 If you are programming with blocks, `interval` is set at 400 milliseconds.
 
-### Example
+## Example
 
 This program shows a picture with the ``show leds`` function.
 
@@ -34,14 +34,14 @@ basic.showLeds(`
 )
 ```
 
-### ~hint
+## ~hint
 
 If you are programming in JavaScript, `#` means an LED that is turned
 on and `.` means an LED that is turned off.
 
-### ~
+## ~
 
-### See also
+## See also
 
 [plot leds](/reference/basic/plot-leds), [show animation](/reference/basic/show-animation)