nixos/doc: improve example of renaming network interfaces

For reliably identifying network interfaces, `PermanentMACAddress` is
likely to be preferable to `MACAddress`. NetworkManager in particular
commonly changes the MAC address of wireless interfaces.

Reference:

  - https://www.freedesktop.org/software/systemd/man/systemd.link.html#PermanentMACAddress=

nixos/doc/manual/configuration/renaming-interfaces.section.md

@@ -26,7 +26,7 @@ we assign the name `wan` to the interface with MAC address
 
 ```nix
 systemd.network.links."10-wan" = {
-  matchConfig.MACAddress = "52:54:00:12:01:01";
+  matchConfig.PermanentMACAddress = "52:54:00:12:01:01";
   linkConfig.Name = "wan";
 };
 ```

nixos/doc/manual/from_md/configuration/renaming-interfaces.section.xml

@@ -32,7 +32,7 @@
     </para>
     <programlisting language="bash">
 systemd.network.links.&quot;10-wan&quot; = {
-  matchConfig.MACAddress = &quot;52:54:00:12:01:01&quot;;
+  matchConfig.PermanentMACAddress = &quot;52:54:00:12:01:01&quot;;
   linkConfig.Name = &quot;wan&quot;;
 };
 </programlisting>