[doc] Arch Linux installation: modified the command for user creation so it...

[doc] Arch Linux installation: modified the command for user creation so it works on both Debian and Arch

docs/installation/debian.rst

@@ -41,7 +41,7 @@ Create the user and the directory:
 
 .. code-block:: shell
 
-    sudo adduser --system --home /srv/funkwhale funkwhale
+    sudo useradd -r -s /usr/bin/nologin -d /srv/funkwhale -m funkwhale
     cd /srv/funkwhale
 
 Log in as the newly created user from now on: