Add unit to vacuum_buffer_usage_limit value in postgresql.conf.sample.

Also adjust the indentation of the comment to the surrounding parameters.

Author: Masahiko Sawada
Reviewed-by: David Rowley, Daniel Gustafsson, Melanie Plageman
Discussion: https://postgr.es/m/CAD21AoCBSqmqOKVH4Q256DeCC_UE50gu1sgixcjLFZGLEbABVA@mail.gmail.com

Author: MasahikoSawada

src/backend/utils/misc/postgresql.conf.sample

@@ -158,9 +158,9 @@
 					#   mmap
 					# (change requires restart)
 #min_dynamic_shared_memory = 0MB	# (change requires restart)
-#vacuum_buffer_usage_limit = 256 # size of vacuum and analyze buffer access strategy ring.
-				 # 0 to disable vacuum buffer access strategy
-				 # range 128kB to 16GB
+#vacuum_buffer_usage_limit = 256kB	# size of vacuum and analyze buffer access strategy ring.
+					# 0 to disable vacuum buffer access strategy
+					# range 128kB to 16GB
 
 # - Disk -