pgsql: Update obsolete comment in ExecScanReScan().

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

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/324051fb890b927a5f85aa899d800f0162952d69

Modified Files
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src/backend/executor/execScan.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)