NGINX: fix typo in defaultBackend PDB
!1002 (merged) was put in to fix #1629 (closed), however there was a typo added due to copy and paste. This was missed at review time
This fixes the property in use, resolving the situation.
Closes #1629 (closed)
cc @twk3 -- for immediate inclusion & tag. I've included 2.1.x here, because ... well. Oops.