Fix typo in Cesium3DTilePass

[jjhembd]

Fixes #11516. There was a copy-paste error in Cesium3DTilePass from #11167 which affected the PRELOAD pass.

[cesium-concierge]

Thanks for the pull request @jjhembd!

• ✔️ 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.

[ggetz]

Awesome, thanks @jjhembd! Please update CHANGES.md with a summary of the fix.

[jjhembd]

@ggetz updated!

[ggetz]

Thanks @jjhembd! CI is failing due to an unrelated travis error, so I'm going to go ahead and merge.