[IndexedDB] Revise references to "default action" (#67)

Per [DOM](https://dom.spec.whatwg.org), "default action" is confusing. Rephrase based on whether the event's __canceled flag__ is set (or whether `preventDefault()` was called)


---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/IndexedDB/issues/67