pgsql: Fix incorrect function name in comments

Fix incorrect function name in comments

Update comments to reference WaitForLSN() instead of the outdated
WaitForLSNReplay() function name.

Discussion: https://postgr.es/m/CABPTF7UieOYbOgH3EnQCasaqcT1T4N6V2wammwrWCohQTnD_Lw%40mail.gmail.com
Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/23792d7381583d04c4bbd0a8507566401ec013d3

Modified Files
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src/backend/commands/wait.c   | 3 +--
src/include/access/xlogwait.h | 2 +-
2 files changed, 2 insertions(+), 3 deletions(-)