add link to purchase reference cards

Author: livialima

docs/00.md

@@ -7,6 +7,8 @@ comments: true
 * Lesson video: [Creating a local VM with VirtualBox](https://www.youtube.com/watch?v=nKcfhI1B4mA)
 * Complementary video: [The many ways to create your server - local VM, AWS, DigitalOcean, and more!](https://youtube.com/live/_-6UYOjRIVQ)
 
+_Help maintain the course by purchasing the [reference card](https://buymeacoffee.com/livialima/e/495370) for this lesson._
+
 ## Requirements
 
 First, you need a server. You can't learn about administering a remote Linux server without having one of your own - so today we're going to get one.

docs/16.md

@@ -23,7 +23,7 @@ Archiving and compressing, however, are actually distinct processes that are fre
 
 Check out the [demo](https://asciinema.org/a/765120)
 
-_Help maintaining the course by purchasing the [reference card](https://buymeacoffee.com/livialima/e/493808) for this lesson._
+_Help maintain the course by purchasing the [reference card](https://buymeacoffee.com/livialima/e/493808) for this lesson._
 
 ## COMPRESSING FILES