doc: Fix type of cursor_position in jsonlog table

This entry was listed as a "string", but it is a "number.  The other
fields are correctly described, on a second look.

Reported-by: Nuko Yokohama
Author: Tatsuo Ishii
Discussion: https://postgr.es/m/CAF3Gu1awoVoDP5d0_eN=cR=QkGVwH+OtFvwJkkc5cB_ZMWjyeA@mail.gmail.com
Backpatch-through: 15

doc/src/sgml/config.sgml

@@ -7801,7 +7801,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         </row>
         <row>
          <entry><literal>cursor_position</literal></entry>
-         <entry>string</entry>
+         <entry>number</entry>
          <entry>Cursor index into query string</entry>
         </row>
         <row>