Added comma to a comment as a trivial tweak

[raovs]

Added a comma to clarify a nonrestrictive clause in a comment as a trivial tweak

/cc @newmuis @gmajoulet

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[amp-owners-bot]

Hey @gmajoulet, these files were changed:

extensions/amp-story/1.0/amp-story.js

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[raovs]

@googlebot I fixed it.

[newmuis]

😂 love the detailed description. This has doubled as a grammar lesson

[gmajoulet]

🥇 🎉