Fix comment for MAX_SIMUL_LWLOCKS.
This comment mentions that pg_buffercache locks all buffer
partitions simultaneously, but it hasn't done so since v10.
Oversight in commit
6e654546fb.
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/aKTuAHVEuYCUmmIy%40nathan