Update obsolete comment in ExecScanReScan().

Commit 27cc7cd2b removed the epqScanDone flag from the EState struct,
and instead added an equivalent flag named relsubs_done to the EPQState
struct; but it failed to update this comment.

Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Discussion: https://postgr.es/m/CAPmGK152zJ3fU5avDT5udfL0namrDeVfMTL3dxdOXw28SOrycg%40mail.gmail.com
Backpatch-through: 13

src/backend/executor/execScan.c

@@ -331,7 +331,7 @@ ExecScanReScan(ScanState *node)
 
 			/*
 			 * If an FDW or custom scan provider has replaced the join with a
-			 * scan, there are multiple RTIs; reset the epqScanDone flag for
+			 * scan, there are multiple RTIs; reset the relsubs_done flag for
 			 * all of them.
 			 */
 			if (IsA(node->ps.plan, ForeignScan))