doc: Use "integer" for some I/O worker GUC type descriptions

doc: Use "integer" for some I/O worker GUC type descriptions

The documentation previously described the io_max_workers,
io_worker_idle_timeout, and io_worker_launch_interval GUCs as
type "int". However, the documentation consistently uses "integer"
for parameters of this type.

This commit updates these parameter descriptions to use "integer"
for consistency.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAHGQGwEpMDpB-K8SSUVRRHg6L6z3pLAkekd9aviOS=ns0EC=+Q@mail.gmail.com