Fix propensity typo in notebook

[pr0ton]

Problem

Typo in notebook.

Proposed changes

• Fix typo in notebook

Types of changes

What types of changes does your code introduce to causaltune?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• Description above provides context of the change
• I have added tests that prove my fix is effective or that my feature works
• Unit tests for changes (not needed for documentation changes)
• Bumping version in setup.py is an individual PR and not mixed with feature or bugfix PRs
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions