Add gin_fuzzy_search_limit to postgresql.conf.sample.

This was forgotten in 8a3631f8 (commit that originally added the parameter)
and 0ca9907c (commit that added the documentation later that year).

Back-patch to all supported versions.

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

@@ -527,6 +527,7 @@
 #bytea_output = 'hex'			# hex, escape
 #xmlbinary = 'base64'
 #xmloption = 'content'
+#gin_fuzzy_search_limit = 0
 #gin_pending_list_limit = 4MB
 
 # - Locale and Formatting -