Fix incorrect comment on how BackendStatusArray is indexed

The comment was copy-pasted from the call to ProcSignalInit() in
AuxiliaryProcessMain(), which uses a similar scheme of having reserved
slots for aux processes after MaxBackends slots for backends. However,
ProcSignalInit() indexing starts from 1, whereas BackendStatusArray
starts from 0. The code is correct, but the comment was wrong.

Discussion: https://www.postgresql.org/message-id/f3ecd4cb-85ee-4e54-8278-5fabfb3a4ed0@iki.fi
Backpatch-through: v14

src/backend/utils/activity/backend_status.c

@@ -263,9 +263,9 @@ pgstat_beinit(void)
 		 * Assign the MyBEEntry for an auxiliary process.  Since it doesn't
 		 * have a BackendId, the slot is statically allocated based on the
 		 * auxiliary process type (MyAuxProcType).  Backends use slots indexed
-		 * in the range from 1 to MaxBackends (inclusive), so we use
-		 * MaxBackends + AuxBackendType + 1 as the index of the slot for an
-		 * auxiliary process.
+		 * in the range from 0 to MaxBackends (exclusive), so we use
+		 * MaxBackends + AuxProcType as the index of the slot for an auxiliary
+		 * process.
 		 */
 		MyBEEntry = &BackendStatusArray[MaxBackends + MyAuxProcType];
 	}