pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/reference/serial/read-line.md

@@ -6,19 +6,19 @@ Read a line of text from the serial port.
 serial.readLine();
 ```
 
-#### ~hint
+### ~hint
 
 This function expects the line it reads to be terminated with the `\r`
 character.  If your terminal software does not terminate lines with
 `\r`, this function will probably never return a value.
 
-#### ~
+### ~
 
-### Returns
+## Returns
 
 * a [string](/types/string) containing input from the serial port, such as a response typed by a user
 
-### Example
+## Example
 
 The following example requests the user's name, then repeats it to greet the user.
 
@@ -31,7 +31,7 @@ basic.forever(() => {
 });
 ```
 
-### See also
+## See also
 
 [serial](/device/serial),
 [serial write line](/reference/serial/write-line),