Fix typo in Control Section of MMM Example notebook

[williambdean]

Description

Related Issue

• Closes #
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Modules affected

• MMM
• CLV

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pymc-marketing--784.org.readthedocs.build/en/784/

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.89%. Comparing base (cec2cc2) to head (0adad52).
Report is 183 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #784   +/-   ##
=======================================
  Coverage   93.89%   93.89%           
=======================================
  Files          28       28           
  Lines        2948     2948           
=======================================
  Hits         2768     2768           
  Misses        180      180           

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[juanitorduz]

I will add this change in #787 🙏