docs: Use hash marks for header lines

Consistently use leading hash marks (#) to note section headers.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/beaglebone.md

@@ -3,8 +3,7 @@
 This document describes the process of running Klipper on a Beaglebone
 PRU.
 
-Building an OS image
---------------------
+## Building an OS image
 
 Start by installing the
 [Debian 9.9 2019-08-03 4GB SD IoT](https://beagleboard.org/latest-images)
@@ -20,8 +19,7 @@ git clone https://github.com/KevinOConnor/klipper
 ./klipper/scripts/install-beaglebone.sh
 ```
 
-Install Octoprint
------------------
+## Install Octoprint
 
 One may then install Octoprint:
 ```
@@ -56,8 +54,7 @@ sudo systemctl start octoprint
 Make sure the octoprint web server is accessible - it should be at:
 [http://beaglebone:5000/](http://beaglebone:5000/)
 
-Building the micro-controller code
-----------------------------------
+## Building the micro-controller code
 
 To compile the Klipper micro-controller code, start by configuring it
 for the "Beaglebone PRU":
@@ -87,15 +84,13 @@ make flash
 sudo service klipper start
 ```
 
-Remaining configuration
------------------------
+## Remaining configuration
 
 Complete the installation by configuring Klipper and Octoprint
 following the instructions in
 [the main installation document](Installation.md#configuring-klipper).
 
-Printing on the Beaglebone
---------------------------
+## Printing on the Beaglebone
 
 Unfortunately, the Beaglebone processor can sometimes struggle to run
 OctoPrint well. Print stalls have been known to occur on complex