doc: Fix datatype for postgres_fdw option
The datatype for analyze_sampling had accidentally been set to text
and not string. Backpatch to v16 where analyze_sampling first was
introduced.
Author: Shinya Kato <Shinya11.Kato@oss.nttdata.com>
Reviewed-by: Laurenz Albe <laurenz.albe@cybertec.at>
Discussion: https://postgr.es/m/
7fd9166b9fda267411793f39986d7f24@oss.nttdata.com
Backpatch-through: v16