Update notebook risk allocation (#1982)

* Update warnings and outputs in MMM allocation assessment

Updated warning messages and output cells in the mmm_allocation_assessment.ipynb notebook to reflect changes in code line numbers and execution context. No changes to core logic or results. Also, minor updates in pymc_marketing/mmm/utility.py (details not shown in diff).

* Update mmm_allocation_assessment.ipynb

Author: cetagostini

docs/source/notebooks/mmm/mmm_allocation_assessment.ipynb

@@ -212,7 +212,7 @@ def _mean_tightness_score(
         samples = _check_samples_dimensionality(samples)
         mean = pt.mean(samples)
         tail_metric = tail_distance(confidence_level)
-        return (mean - alpha * tail_metric(samples, budgets)) / mean
+        return 1 - alpha * tail_metric(samples, budgets) / mean
 
     return _mean_tightness_score