Fix comment for ExecProcessReturning

resultRelInfo is the argument for the function, not projectReturning.

Author: Etsuro Fujita
Discussion: https://postgr.es/m/5AA8E11E.1040609@lab.ntt.co.jp

src/backend/executor/nodeModifyTable.c

@@ -141,7 +141,7 @@ ExecCheckPlanOutput(Relation resultRel, List *targetList)
 /*
  * ExecProcessReturning --- evaluate a RETURNING list
  *
- * projectReturning: RETURNING projection info for current result rel
+ * resultRelInfo: current result rel
  * tupleSlot: slot holding tuple actually inserted/updated/deleted
  * planSlot: slot holding tuple returned by top subplan node
  *