fix(InstagramEmbed): position slices for caption content *inside* the embed

[tim-evans]

Tests were previously incorrectly having the slice of the instagram embed run over the embed itself. I failed to catch this in the snapshot tests, but encountered this when testing embedding by pasting in the Instagram embed code directly into a text editor and it rendering as nothing. 😭

Relevant lines that fix the code are:

https://github.com/CondeNast/atjson/blob/ad4d15371d997a4978879588e9f90e796c5b7adb/packages/%40atjson/source-html/src/converter/social-embeds.ts