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: MasaoFujii

doc/src/sgml/config.sgml

@@ -2788,7 +2788,7 @@ include_dir 'conf.d'
       </varlistentry>
 
       <varlistentry id="guc-io-workers" xreflabel="io_workers">
-       <term><varname>io_workers</varname> (<type>int</type>)
+       <term><varname>io_workers</varname> (<type>integer</type>)
        <indexterm>
         <primary><varname>io_workers</varname> configuration parameter</primary>
        </indexterm>