Minor typos in mathematical definitions in intro_to_gps.py

[neel-maniar]

Type of changes

• [ ] Bug fix
• [ ] New feature
• [x] Documentation / docstrings
• [ ] Tests
• [ ] Other

Checklist

• [ ] I've formatted the new code by running poetry run pre-commit run --all-files --show-diff-on-failure before committing.
• [ ] I've added tests for new code.
• [ ] I've added docstrings for the new code.

Description

The definition of joint distribution was missing a line of the covariance matrix and the definition of marginalisation was missing the end of a line as well.

Issue Number: N/A