pgsql: Fix ReorderBufferCheckMemoryLimit() comment.

Fix ReorderBufferCheckMemoryLimit() comment.

Commit 7259736a6 updated the comment but it was not correct since
ReorderBufferLargestStreamableTopTXN() returns only top-level
transactions.

Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CAD21AoA9XB7OR86BqvrCe2dMYX%2BZv3-BvVmjF%3DGY2z6jN-kqjg%40mail.gmail.com
Backpatch-through: 14

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/01357f1da94b9926336448d88ac519f20ab27cd7

Modified Files
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src/backend/replication/logical/reorderbuffer.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)