use URL for README image

[wd60622]

Description

Use URLs instead of relative paths in README should make the PyPI page a bit nicer 🎉

Related Issue

• Closes Relative path doesn't show image on PyPI #491
• 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):

---

📚 Documentation preview 📚: https://pymc-marketing--715.org.readthedocs.build/en/715/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.57%. Comparing base (7be19f6) to head (2961f5b).
Report is 112 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #715   +/-   ##
=======================================
  Coverage   92.57%   92.57%           
=======================================
  Files          24       24           
  Lines        2520     2520           
=======================================
  Hits         2333     2333           
  Misses        187      187           

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