Comment out autovacuum_worker_slots in postgresql.conf.sample.

All settings in this file should be commented out.  In addition to
fixing that, also fix the indentation for this line.

Oversight in commit c758119e5b.

Reported-by: Daniel Gustafsson <daniel@yesql.se>
Author: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/19727040-3EE4-4719-AF4F-2548544113D7%40yesql.se
Backpatch-through: 18

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

@@ -688,7 +688,7 @@
 
 #autovacuum = on			# Enable autovacuum subprocess?  'on'
 					# requires track_counts to also be on.
-autovacuum_worker_slots = 16	# autovacuum worker slots to allocate
+#autovacuum_worker_slots = 16		# autovacuum worker slots to allocate
 					# (change requires restart)
 #autovacuum_max_workers = 3		# max number of autovacuum subprocesses
 #autovacuum_naptime = 1min		# time between autovacuum runs