Obsolete comment in ExecScanReScan()

While working on a patch to add more test coverage for EPQ testing
posted right before, I noticed $SUBJECT:

            /*
             * If an FDW or custom scan provider has replaced the join with a
             * scan, there are multiple RTIs; reset the epqScanDone flag for
             * all of them.
             */

We removed epqScanDone and instead added relsubs_done in commit
27cc7cd2b, so the second part of the comment should be something like
this: "reset the relsubs_done flag for all of them."  Attached is a
patch for that.

Best regards,
Etsuro Fujita