docs - can set writable_external_table_bufize at session level (#4569)

gpdb-doc/dita/ref_guide/config_params/guc-list.xml

@@ -9736,7 +9736,7 @@
             <row>
               <entry colname="col1">integer 32 - 131072 (32KB - 128MB)</entry>
               <entry colname="col2">64</entry>
-              <entry colname="col3">local<p>system</p><p>reload</p></entry>
+              <entry colname="col3">local<p>session</p><p>reload</p></entry>
             </row>
           </tbody>
         </tgroup>