doc: Fix table cell overflow

doc: Fix table cell overflow

Fix one instance of a table cell overflow by adding a zero-width
space.  The visual impact of this is minimal, but since this is
currently the only such case reported by FOP ("contents of ... exceed
the available area"), it seems worth getting rid of.

Back-patch of v14-era commit 74ff28197 into v13.  It's not obvious
to me why we were not seeing this warning in v13 before, since this
file hasn't changed recently.  Nonetheless, it's there now, and it's
still desirable to keep v13 and up clean on this score.