Fix stats_fetch_consistency default value indicated in postgresql.conf.sample.

Mistake in 5891c7a8ed, likely made when switching the default value from none
to fetch during development.

Reported-By: Nathan Bossart <nathandbossart@gmail.com>
Author: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/20220524220147.GA1298892@nathanxps13

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

@@ -619,7 +619,7 @@
 #track_io_timing = off
 #track_wal_io_timing = off
 #track_functions = none			# none, pl, all
-#stats_fetch_consistency = none
+#stats_fetch_consistency = cache
 
 
 # - Monitoring -