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 53c2a97a92.  08b9b9e043 has fixed a similar
inconsistency some time ago.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/aHdxN0D0hKXzHFQG@ip-10-97-1-34.eu-west-3.compute.internal
Backpatch-through: 17

src/backend/storage/lmgr/lwlock.c

@@ -162,8 +162,8 @@ static const char *const BuiltinTrancheNames[] = {
 	[LWTRANCHE_DSM_REGISTRY_DSA] = "DSMRegistryDSA",
 	[LWTRANCHE_DSM_REGISTRY_HASH] = "DSMRegistryHash",
 	[LWTRANCHE_COMMITTS_SLRU] = "CommitTsSLRU",
-	[LWTRANCHE_MULTIXACTOFFSET_SLRU] = "MultixactOffsetSLRU",
-	[LWTRANCHE_MULTIXACTMEMBER_SLRU] = "MultixactMemberSLRU",
+	[LWTRANCHE_MULTIXACTOFFSET_SLRU] = "MultiXactOffsetSLRU",
+	[LWTRANCHE_MULTIXACTMEMBER_SLRU] = "MultiXactMemberSLRU",
 	[LWTRANCHE_NOTIFY_SLRU] = "NotifySLRU",
 	[LWTRANCHE_SERIAL_SLRU] = "SerialSLRU",
 	[LWTRANCHE_SUBTRANS_SLRU] = "SubtransSLRU",