nixos manual: fix syntax

It appears I've cherry-picked wrong commit without this fix.

Changed files
+1 -1
nixos
doc
manual
configuration
+1 -1
nixos/doc/manual/configuration/x-windows.xml
···
</simplesect>
-
<simplesect><title>GTK/Qt themes</title></simplesect>
+
<simplesect><title>GTK/Qt themes</title>
<para>GTK themes can be installed either to user profile or system-wide (via
<literal>system.environmentPackages</literal>). To make Qt 5 applications look similar