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