nixos/acme: remove doc note about restarting nginx

Discussion from #30945 indicates that it is no longer true that you have
to restart nginx.

nixos/modules/security/acme.xml

···
   };
 }
 </programlisting>
-
-<para>At the moment you still have to restart Nginx after the ACME
-certs arrive.</para>
 </section>
 </chapter>