This commit fixes wrong wording like "a fewer kinds"
in the description about track_planning option.

Back-patch to v13 where pg_stat_statements.track_planning was added.

Author: Justin Pryzby
Reviewed-by: Julien Rouhaud, Fujii Masao
Discussion: https://postgr.es/m/20210418233615.GB7256@telsasoft.com