Fix lwlock.c and wait_event_names.txt discrepancy

Hi hackers,

While working on [1], I noticed a discrepancy between lwlock.c and
wait_event_names.txt for 2 wait events.

Indeed, the wait event names are MultixactOffsetSLRU and MultixactMemberSLRU in
the C file but MultiXactOffsetSLRU and MultiXactMemberSLRU in the text file.

That breaks joins between pg_stat_activity and pg_wait_events on the wait
event name. PFA a patch to fix those (I'm not able to find other discrepancy
with the "automated" work I'm doing in [1], so I'm confident there is no others).

Oversight in commit 53c2a97a9266.

[1]: https://www.postgresql.org/message-id/aHZiHZ8sSQdHpyM6%40bdt-Laptop-13th-Gen-Intel-Core

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com