Proper Arch Linux spelling

It's either Arch Linux or archlinux but never Archlinux or ArchLinux.

docs/content/documentation/getting-started/installation.md

@@ -22,7 +22,7 @@ Gutenberg is available on [Scoop](http://scoop.sh):
 $ scoop install gutenberg
 ```
 
-## Archlinux
+## Arch Linux
 
 Use your favourite AUR helper to install the `gutenberg-bin` package.