pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/reference/serial/read-string.md

@@ -6,11 +6,11 @@ Read the buffered serial data as a string
 serial.readString();
 ```
 
-### Returns
+## Returns
 
 * a [string](/types/string) containing input from the serial port. Empty if no data available.
 
-### Example
+## Example
 
 The following program scrolls text on the screen as it arrives from serial.
 
@@ -20,7 +20,7 @@ basic.forever(() => {
 });
 ```
 
-### See also
+## See also
 
 [serial](/device/serial),
 [serial write line](/reference/serial/write-line),