docs: update superuser username in the getting_started.rst file

[hsinkoff]

Update the docs to include the correct email domain for the username.

I've completed each of the following or determined they are not applicable:

• Made a plan to communicate any major developer interface changes (or N/A)
• N/A

[openedx-webhooks]

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[itsjeyd]

Hey @hsinkoff, it looks like the PR bot marked this as an open source contribution, and I'm wondering if that's correct (based on the fact that you're a member of both https://github.com/edx and https://github.com/openedx)?

Please let me know if you think we can remove the label.

CC @mphilbrick211

[hsinkoff]

itsjeyd,

I believe that the open-source-contribution label can be removed. @jristau1984 is helping me get added to the correct group so that will not show up on future PRs.

[itsjeyd]

@hsinkoff OK, I removed the label.

[rgraber]

LGTM

[rgraber]

Not sure why it's failing the Mac ci check. Checking with arch-bom.