Fix typo in BSK principles documentation

[otler17]

Fixed a typo in the file bskPrinciples-0.rst

[schaubh]

Thanks for the typo fix. The CI online tests are failing as this file has not been run through the pre-commit checks. See
https://github.com/AVSLab/basilisk/blob/develop/CONTRIBUTING.md
for instructions on how to do this.

[schaubh]

You are touching an old file that hasn't been edited in a while. Any new commits must follow the style guidelines which can require fixing prior items. The error message is

Fix End of Files.........................................................Failed
- hook id: end-of-file-fixer
- exit code: 1
- files were modified by this hook

Fixing docs/source/Learn/bskPrinciples/bskPrinciples-0.rst

This file has two blank lines at the end, and the format expects a single blank line. Just delete one blank line, commit that change. Then the CI tests should pass.

[schaubh]

Fixed the typo now in another branch.