doc: cosmetic changes in index items

Hi,

I found that only the following three index items have " ,"
(i.e., space + comma) in the docs. This is not harmful and
is very minor issue, but I'd like to get rid of that unnecessary
space character for the sake of consistency. Thought?
Patch attached.

parallel_leader_participation configuration parameter , Other Planner Options
pg_trgm.strict_word_similarity_threshold configuration parameter , GUC Parameters
pg_trgm.word_similarity_threshold configuration parameter , GUC Parameters

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION