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

Author: danielgustafsson

doc/src/sgml/postgres-fdw.sgml

@@ -337,7 +337,7 @@ OPTIONS (ADD password_required 'false');
    <variablelist>
 
     <varlistentry>
-     <term><literal>analyze_sampling</literal> (<type>text</type>)</term>
+     <term><literal>analyze_sampling</literal> (<type>string</type>)</term>
      <listitem>
       <para>
        This option, which can be specified for a foreign table or a foreign