Fix malformed equation in MMM docstring

[wd60622]

The MMM linear model equation was not displaying correctly

@juanitorduz is varepsilon a typo and a math variable? Should it just be epsilon instead?

📚 Documentation preview 📚: https://pymc-marketing--1085.org.readthedocs.build/en/1085/

Check the rendering: https://pymc-marketing--1085.org.readthedocs.build/en/1085/api/generated/pymc_marketing.mmm.mmm.MMM.html#pymc_marketing.mmm.mmm.MMM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.56%. Comparing base (95bc1fa) to head (95084c6).
Report is 112 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1085   +/-   ##
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  Coverage   95.56%   95.56%           
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  Files          39       39           
  Lines        4011     4011           
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  Hits         3833     3833           
  Misses        178      178           

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[juanitorduz]

I like varepsilon more haha https://tex.stackexchange.com/questions/98013/varepsilon-vs-epsilon

[juanitorduz]

Thanks @wd60622