Update comment for IndexInfo.ii_NullsNotDistinct

Commit 7a7b3e1 added the ii_NullsNotDistinct field, but the
comment was not updated.

Author: Japin Li <[email protected]>
Reviewed-by: Richard Guo <[email protected]>
Discussion: https://www.postgresql.org/message-id/flat/ME0P300MB04453E6C7EA635F0ECF41BFCB6832%40ME0P300MB0445.AUSP300.PROD.OUTLOOK.COM

Author: petere

src/include/nodes/execnodes.h

@@ -173,6 +173,7 @@ typedef struct ExprState
  *		UniqueProcs
  *		UniqueStrats
  *		Unique				is it a unique index?
+ *		NullsNotDistinct	is NULLS NOT DISTINCT?
  *		ReadyForInserts		is it valid for inserts?
  *		CheckedUnchanged	IndexUnchanged status determined yet?
  *		IndexUnchanged		aminsert hint, cached for retail inserts