Fix inconsistent LWLock tranche names for MultiXact*

Fix inconsistent LWLock tranche names for MultiXact*

The terms used in wait_event_names.txt and lwlock.c were inconsistent
for MultiXactOffsetSLRU and MultiXactMemberSLRU, which could cause joins
between pg_wait_events and pg_stat_activity to fail.  lwlock.c is
adjusted in this commit to what the historical name of the event has
always been, and what is documented.

Oversight in 53c2a97a9266.  08b9b9e043bb has fixed a similar
inconsistency some time ago.

Author: Bertrand Drouvot <[email protected]>
Reviewed-by: Álvaro Herrera <[email protected]>
Discussion: https://postgr.es/m/[email protected]
Backpatch-through: 17