Fix properties assigned to wrong object

[jjspace]

Description

Two properties in the Terrain object were set as @memberOf Viewer.prototpye instead of on the Terrain object. I believe this is just a fix of docs and not missing implementation, @ggetz please confirm?

Issue number and link

Fixes #11932

Testing plan

• run npm run build-docs
• npm start
• Check the Viewer docs locally and make sure provider and ready are not defined
• Check the Terrain docs locally and make sure they do show up there

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 update 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.

[ggetz]

Yes, thanks for the fix @jjspace!