Change "in a frame" involving "oEmbed" to "in an iframe"

Problem/Motivation

A followup of #2851394: Fix grammar 'a' to 'an' when necessary → , pointed by @longwave in #259 → , as there is a UI change, tagging "Needs usability review"

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet