Add ES_PREFIX in .env.production.sample (#10087)

.env.production.sample

@@ -10,6 +10,7 @@ DB_NAME=postgres
 DB_PASS=
 DB_PORT=5432
 # Optional ElasticSearch configuration
+# You may also set ES_PREFIX to share the same cluster between multiple Mastodon servers (falls back to REDIS_NAMESPACE if not set)
 # ES_ENABLED=true
 # ES_HOST=es
 # ES_PORT=9200