Update CodeSandbox link in README

[paulhibbitts]

Summary

Related issue, if any:

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

For any code change,

• Related documentation has been updated, if needed
• Related tests have been added or updated, if needed

Does this PR introduce a breaking change?

• Yes
• No

Tested in the following browsers:

• Chrome
• Firefox
• Safari
• Edge

[vercel]

@paulhibbitts is attempting to deploy a commit to the Docsify Team on Vercel.

A member of the Team first needs to authorize it.

[sy-records]

It also needs to be modified here.

https://github.com/docsifyjs/docsify/blob/develop/.github/ISSUE_TEMPLATE/bug_report.yml?plain=1#L36

But I think the link is too long. I plan to add a custom domain to jump to it. For example, sandbox.docsifyjs.org

[paulhibbitts]

I can further shorten the link name I think, just let me know if you'd like that done. Feel free to fork of course as well if easier etc.