doc: Fix description of the JIT time fields in pg_stat_statements
The four fields tracking the time spent by queries for JIT operations
added in
57d6aea were listed as having bigint as data type, but these
are of type double precision.
Author: Noriyoshi Shinoda
Reviewed-by: Nathan Bossart
Discussion: https://postgr.es/m/DM4PR84MB1734375E63148AADC6A1E6B4EED39@DM4PR84MB1734.NAMPRD84.PROD.OUTLOOK.COM