doc: Fix type description of io_workers GUC for consistency.

doc: Fix type description of io_workers GUC for consistency.

The documentation previously described the type of the io_workers GUC
parameter as "int". However, the documentation consistently uses "integer"
for parameters of this type.

This commit updates the type description of io_workers to "integer"
for consistency with other GUC parameter descriptions.

Author: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/e95c899a-2aeb-45b7-8fd3-7a27dcdb475b@oss.nttdata.com