Hide time series + xgboost demos by default

[freddyaboulton]

Description

Fix a mistake I made in #2067 where I didn't hide the new demos by default

Checklist:

• I have performed a self-review of my own code
• My code follows the style guidelines of this project
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[aliabd]

The link for the source on time series forecasting is broken, missing the /spaces/.

Also I would recommend adding a code block like the original demos, and linking to the code there instead, something like below, but definitely not a big deal.

[freddyaboulton]

Thanks for pointing out the broken link @aliabd 🙈 ! Will fix in the space + make the other changes we discussed offline (replacing source with code and linking to the code itself)