README: mention official Arch Linux package (#226)

Signed-off-by: Christian Rebischke <chris@nullday.de>
Co-authored-by: Filippo Valsorda <1225294+FiloSottile@users.noreply.github.com>

README.md

@@ -78,12 +78,10 @@ go build -ldflags "-X main.Version=$(git describe --tags)"
 
 or use [the pre-built binaries](https://github.com/FiloSottile/mkcert/releases).
 
-For Arch Linux users, mkcert is available from AUR as [`mkcert`](https://aur.archlinux.org/packages/mkcert/) or [`mkcert-git`](https://aur.archlinux.org/packages/mkcert-git/).
+For Arch Linux users, [`mkcert`](https://www.archlinux.org/packages/community/x86_64/mkcert/) is available on the official Arch Linux repository.
 
-```bash
-git clone https://aur.archlinux.org/mkcert.git
-cd mkcert
-makepkg -si
+```
+sudo pacman -Syu mkcert
 ```
 
 ### Windows