Rename `event` to `action` in HAML Snowplow helper

In the Snowplow Guide, we talk about how to fire events from a HAML file:

%button.btn{ data: { track: { event: "click_button", label: "template_preview", property: "my-template" } } }

The term event is somewhat ambiguous here, and is referring to the action attribute of the event that will be sent. In order to make this clearer, we should rename event to action.

Note that action is referring to an attribute in the Structured Event Taxonomy.