[Tree Building Exercise & Class Inheritance Concept] Spelling Fixes

[MarkRosemaker]

This PR fixes spelling errors. Namely, possessive 'its' spelling in documentation and tests.

Unfortunately, one of the possessive 'its' was misspelled in a test, forcing every student to repeat that mistake in order to successfully submit the exercise.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!

[BethanyG]

Hi @MarkRosemaker 👋🏽

Thanks for the PR. 😄 Per my note on the forum, please remove the changes to the reference directory. After that, I can review. Thanks!

[BethanyG]

Thanks for this! 🎉 🚀