Fixed "Bing Maps Road" option in sandcastle example

[ebogo1]

Fixes #7524.

@ptrgags

[cesium-concierge]

Thank you so much for the pull request @ebogo1! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

The Pull Request Guidelines is a handy reference for making sure your PR gets accepted quickly, so make sure to skim that.

• ❌ Missing CONTRIBUTORS.md entry.
  • Please add yourself to the contributors file!
• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[ptrgags]

This looks good to me. @lilleyse and I decided we're going to merge this once CI passes since it's a tiny fix.

@ebogo1's contributors entry is in his other branch that'll be merged post-release.

[ptrgags]

Looks good now, thanks @ebogo1!