Use second level headers on notes page

docs/notes.md

@@ -1,6 +1,6 @@
 # Notes
 
-1. **BCM pin numbering**
+## BCM pin numbering
 
 This library uses Broadcom (BCM) pin numbering for the GPIO pins, as
 opposed to BOARD. Unlike the `RPi.GPIO` library, this is not configurable.
@@ -37,18 +37,20 @@
 - *5V = 5 Volts*
 - *DNC = Do not connect (special use pins)*
 
-2.  **Wiring**
+## Wiring
 
 All components must be wired up correctly before using with this library.
 
-3.  **Keep your program alive with `signal.pause`**
+## Keep your program alive with `signal.pause`
 
 The following program looks like it should turn an LED on:
 
+```python
 from gpiozero import led
 
 led = LED(2)
 led.on()
+```
 
 And it does, if you're using the Python shell, IPython shell or IDLE shell,
 but if you saved this program as a Python file and ran it, it would flash
@@ -57,12 +59,14 @@
 The following file includes an intentional `pause` to keep the program
 alive:
 
+```python
 from gpiozero import LED
 from signal import pause
 
 led = LED(2)
 led.on()
 pause()
+```
 
 Now running the program will stay running, leaving the LED on, until it is
 forced to quit.
@@ -70,9 +74,11 @@
 Similarly, when setting up callbacks on button presses or other input
 devices, the program needs to be running for the events to be detected:
 
+```python
 from gpiozero import Button
 from signal import pause
 
 button = Button(2)
 button.when_pressed = lambda: print("Button was pressed!")
 pause()
+```