Changed the name of few structure members for the sake of clarity and
removed spurious whitespace.

Reported-by: Amit Kapila
Author: Amit Kapila, based on suggestion by Andrew Dunstan
Reviewed-by: Alvaro Herrera
Discussion: https://postgr.es/m/CAA4eK1K2znsFpC+NQ9A4vxT4uDxADN4RmvHX0L6Y=aHVo9gB4Q@mail.gmail.com