Documentation on topic handler kind email should say smtp

Following instructions from [kapacitor 1.5 docs](https://docs.influxdata.com/kapacitor/v1.5/event_handlers/email/#example-handler-file) I have received an error stating that action kind email is wrong. The correct kind seems to be `smtp` and I think documentation should reflect that.

Start with the linked Kapacitor 1.5 email event-handler documentation, especially the example handler file. Change the documented action kind from `email` to `smtp`, and confirm the example no longer directs users toward the reported error.

Written by the indexing model from the issue text.