nixos/manual: fix build

Broken due to a43e33d0e48b2284ac3a2222d7f1965cef66f5e2.

nixos/doc/manual/release-notes/rl-1803.xml

···
     </para>
     <itemizedlist>
       <listitem>
-         <literal>sound.enable</literal> now defaults to false.
+        <para>
+          <literal>sound.enable</literal> now defaults to false.
+        </para>
       </listitem>
       <listitem>
         <para>