Closing pop-up window in search for places plant name is used: tweak for XProc display

[LucasHorseshoeBend]

There is an inconsistency in the mechanism for closing a pop up window
Compare the footnote window and the search for plant name windows in screen shot.
Not fatal, but better to be consistent, using the closing X symbol which is much more intuitive.

Not urgent, but would be good to do it next time the interface is updated.