nixos/stage-1: update udev.log_level name in docs

I was confused why I couldn't find a mention of udev.log_priority in
systemd-udevd.service(8).  It turns out that it was renamed[1] to
udev.log_level.  The old name is still accepted, but it'll avoid
further confusion if we use the new name in our documentation.

[1]: 64a3494c3d

nixos/modules/system/boot/stage-1.nix

@@ -633,7 +633,7 @@ in
 
           <itemizedlist>
             <listitem><para><literal>boot.consoleLogLevel = 0;</literal></para></listitem>
-            <listitem><para><literal>boot.kernelParams = [ "quiet" "udev.log_priority=3" ];</literal></para></listitem>
+            <listitem><para><literal>boot.kernelParams = [ "quiet" "udev.log_level=3" ];</literal></para></listitem>
           </itemizedlist>
         '';
     };