pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/reference/led/set-brightness.md

@@ -7,20 +7,20 @@ turned on.
 led.setBrightness(121)
 ```
 
-### Parameters
+## Parameters
 
 * ``value`` is a [number](/types/number) that means how
   bright the screen is when it is turned on, from `0` (darkest) to
   `255` (brightest). For example, the number `127` means the screen is
   halfway bright when it is turned on.
 
-### ~ hint
+## ~ hint
 
 The brightness is not supported in the simulator. You will need to try it on the @boardname@ itself!
 
-### ~
+## ~
 
-### Example: change brightness
+## Example: change brightness
 
 This program makes the screen brightness 100% (`255`).  Then it turns on
 the center LED (`2, 2`), waits for one second, and then sets the screen
@@ -33,6 +33,6 @@ basic.pause(1000)
 led.setBrightness(led.brightness() / 2)
 ```
 
-### See also
+## See also
 
 [brightness](/reference/led/brightness), [fade in](/reference/led/fade-in), [fade out](/reference/led/fade-out), [LED screen](/device/screen)