Update quickstart.md

Created by: Kelvin-Lee

Colleagues and I could not get the original run step of the example batch spec to work as expected. Our discussion is here, in a Slack thread.

Changing the run step to use sed instead of tee.

Updated wording a bit.

As far as I skimmed, the screenshots that are in the doc will still work/make sense even with the proposed change.

Test plan