fixed typo: Introduction to Trees

[nshah1729]

Fixes #

Short description of what this resolves:

What Changes proposed in this pull request:

-Corrected a Typo

Checklist

• [x] PR title follows semantic commit guidelines
• [x] Search previous suggestions before making a new one, as yours may be a duplicate.
• [x] I acknowledge that all my contributions will be made under the project's license.

Notes: