fix typo in user creation

README.md

@@ -118,7 +118,7 @@ that case you will need to change the path in the `.service` file.
 Then add a role user for the project, grant it access to the Docker
 daemon:
 
-    adduser --systemd dangerzone
+    adduser --system dangerzone
     adduser dangerzone docker
 
 If you want to use another username, you will need to modify the