Fix error message in hub.py

[srai9]

What does this PR do?

This PR fixes a broken link in an error message in hub.py. A space is missing between the link and the following text.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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@stevhliu @Rocketknight1

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[stevhliu]

Thanks for fixing!

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.