Fix pointer type in ExecForeignBatchInsert SGML docs

Reported-by: Ian Barwick
Discussion: https://postgr.es/m/20200628151002.7x5laxwpgvkyiu3q@development

Author: tvondra

doc/src/sgml/fdwhandler.sgml

@@ -619,7 +619,7 @@ TupleTableSlot **
 ExecForeignBatchInsert(EState *estate,
                   ResultRelInfo *rinfo,
                   TupleTableSlot **slots,
-                  TupleTableSlot *planSlots,
+                  TupleTableSlot **planSlots,
                   int *numSlots);
 </programlisting>