README: fixed broken readme link to arch linux package (#580)

README.md

@@ -83,7 +83,7 @@ chmod +x mkcert-v*-linux-amd64
 sudo cp mkcert-v*-linux-amd64 /usr/local/bin/mkcert
 ```
 
-For Arch Linux users, [`mkcert`](https://www.archlinux.org/packages/community/x86_64/mkcert/) is available on the official Arch Linux repository.
+For Arch Linux users, [`mkcert`](https://archlinux.org/packages/extra/x86_64/mkcert/) is available on the official Arch Linux repository.
 
 ```
 sudo pacman -Syu mkcert