chore(docs) fix example value (#849)

Use "expressions" instead of "expression"
kong.conf expects the former.

charts/kong/values.yaml

@@ -86,7 +86,7 @@ env:
   database: "off"
   # the chart uses the traditional router (for Kong 3.x+) because the ingress
   # controller generates traditional routes. if you do not use the controller,
-  # you may set this to "traditional_compatible" or "expression" to use the new
+  # you may set this to "traditional_compatible" or "expressions" to use the new
   # DSL-based router
   router_flavor: "traditional"
   nginx_worker_processes: "2"