Updated systemd caddy.service (#2620)

According to https://github.com/systemd/systemd/blob/v241/NEWS#L2799
The Directive ReadWriteDirectories= has been renamed to ReadWritePaths=
in 241.

dist/init/linux-systemd/caddy.service

@@ -38,7 +38,7 @@ ProtectHome=true
 ProtectSystem=full
 ; … except /etc/ssl/caddy, because we want Letsencrypt-certificates there.
 ;   This merely retains r/w access rights, it does not add any new. Must still be writable on the host!
-ReadWriteDirectories=/etc/ssl/caddy
+ReadWritePaths=/etc/ssl/caddy
 
 ; The following additional security directives only work with systemd v229 or later.
 ; They further restrict privileges that can be gained by caddy. Uncomment if you like.