fixing various lessons menu

docs/lessons/glowing-pendulum/challenges.md

@@ -24,13 +24,13 @@ basic.forever(() => {
 
 ```
 
-**Challenge 1**
+### Challenge 1
 
 ![](/static/mb/lessons/glowing-pendulum-0.jpg)
 
 Hold the micro:bit in your hand in a dark room. Move the micro:bit like a pendulum and produce a slow image that captures the pattern of the micro:bit LEDs.
 
-**Challenge 2**
+### Challenge 2
 
 Replace "y" in `acceleration(y)` with "x" or "z". Changing the axis will cause the micro:bit to measure the force in a different direction. What differences in the resulting pattern does this replacement make?