Update note on helm chart nodeports

**What**
- Update the readme to reflect that only the Gateway has a NodePort by
default.

**Why**
- The prometheus component should not be exposed because it is an
internal implementation detail used to control scaling etc.  Changing
the docs better reflects this role and behavior

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

chart/openfaas/README.md

@@ -135,7 +135,7 @@ $ helm upgrade --install openfaas openfaas/ \
 
 ### NodePorts
 
-By default NodePorts will be created for the API Gateway and Prometheus.
+By default a NodePort will be created for the API Gateway.
 
 ### LB