Fix range for commit_siblings in sample conf

The range for commit_siblings was incorrectly listed as starting on 1
instead of 0 in the sample configuration file.  Backpatch down to all
supported branches.

Author: Man Zeng <zengman@halodbtech.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/tencent_53B70BA72303AE9C6889E78E@qq.com
Backpatch-through: 14

Author: danielgustafsson

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

@@ -256,7 +256,7 @@
 #wal_skip_threshold = 2MB
 
 #commit_delay = 0			# range 0-100000, in microseconds
-#commit_siblings = 5			# range 1-1000
+#commit_siblings = 5			# range 0-1000
 
 # - Checkpoints -