Add missing semicolon (#63919)

Signed-off-by: Matthias Beyer <[e-mail]>

nixos/doc/manual/configuration/ipv6-config.xml

···
 <xref linkend="opt-networking.defaultGateway6"/> = {
   address = "fe00::1";
   interface = "enp0s3";
-}
+};
 </programlisting>
  </para>