doc: Get rid of unnecessary space character from some index items.

doc: Get rid of unnecessary space character from some index items.

Previously some index items have " ," (i.e., space + comma) in the docs
as follows. Since the space character before the comma is unnecessary,
this commit gets rid of that for the sake of consistency with other
index items.

   parallel_leader_participation configuration parameter , Other Planner Options

Author: Fujii Masao
Reviewed-by: Euler Taveira
Discussion: https://postgr.es/m/e87b4ddf-1498-2850-bf55-519df3928fd4@oss.nttdata.com