Typo in adding upstream repository.

[ckarampa]

The URL was ".../YOUR_USERNAME/.." instead of ".../ORIGINAL_OWNER/..." in the part where adding the upstream repository is explained.

Why:

• URL was misleading

Closes #17785

What's being changed:

• Fixed one typo in the example where the upstream repository is added.

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Content directory changes

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Source	Preview	Production	What Changed
get-started/quickstart/fork-a-repo.md	fpt ghec ghes@ 3.5 3.4 3.3 3.2 3.1 ghae	fpt ghec ghes@ 3.5 3.4 3.3 3.2 3.1 ghae

[ramyaparimi]

@ckarampa
Thanks so much for opening a PR! I'll get this triaged for review ⚡

[skedwards88]

Thanks for catching this! We'll get this merged down for you.

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