doc: fix udev(7) documentation about ATTR{} and SYSCTL{}

doc: fix udev(7) documentation about ATTR{} and SYSCTL{}

Without this fix, udev(7) munges the two items together, like so:

       ATTR{filename}, SYSCTL{kernel parameter}
           Match sysfs attribute values of the event device. Trailing
           whitespace in the attribute values is ignored unless the specified
           match value itself contains trailing whitespace.  Match a kernel
           parameter value.

You're not allowed to have a <term> element after a <listitem> element within a
<varlistentry>.

xmllint complains:

    udev.xml:192: element varlistentry: validity error : Element varlistentry content does not follow the DTD, expecting (term+ , listitem), got (term listitem term listitem )