Update upgrade-ubuntu.md

fix: lint issue

Author: ahoneybun

content/upgrade-ubuntu.md

@@ -27,10 +27,9 @@ First, we recommend having Ubuntu installation media (a DVD or USB drive) handy
 ## Create Install Media
 
 Create an Installation USB
---------------------------
-[Using Ubuntu](http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-ubuntu)
-[Using Windows](http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-windows)
-[Using Mac OS X](http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-mac-osx)
+- [Using Ubuntu](http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-ubuntu)
+- [Using Windows](http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-windows)
+- [Using Mac OS X](http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-mac-osx)
 
 Next, we recommend backing up all important files from your computer. The upgrade process will leave your files intact, but it's best to be safe in case something goes wrong. We have a support article that goes over creating backups [here.](/articles/backup-files)