Fix sandcastle contributors guide link

[jjspace]

Description

This is a very small edgecase scenario that happens when someone tries to open a sandcastle html file directly instead of using the local server. The page it was trying to direct to is on the Wiki we don't really use anymore and instead tried to create a page (if you have access) or redirects to the home page instead of the info you need.
There was one other instance of this check that did have a valid link so this just updates it to match that one

cesium/Apps/Sandcastle/index.html

Lines 36 to 37 in fddd0f1

	window.location =
	"https://github.com/CesiumGS/cesium/blob/main/Documentation/Contributors/BuildGuide/README.md#quickstart";

Issue number and link

No issue

Testing plan

• Open the Sandcastle gallery in your file explorer
• Open any html file directly
• You should see an alert about using the local server and when clicking OK it should redirect you to the contributor docs.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjspace!

✅ We can confirm we have a CLA on file for you.

[lukemckinstry]

What do you mean by this step?

Open any html file directly

[jjspace]

Open the file directly, like double click it and open in a browser not loading it from the localhost server

[jjhembd]

Here's what I see when I load one of the HTMLs from Windows File Explorer:

If I click on "OK" then it takes me here:

Looks good to me!