Fixed aggregation issue in grouping

[mwael-labs]

This PR updates the Step 4 solution in the Pandas exercises 03_Grouping -> Regiment.
The original code worked in older Pandas versions (pre 1.0) but now raises a TypeError in modern Pandas (>= 1.5, 2.0+) because of stricter handling of non‑numeric columns during aggregation.

Problem

regiment[regiment['regiment'] == 'Nighthawks'].groupby('regiment').mean()

Old behaviour ignored non-numeric columns.
New behaviour attempts to aggregate all columns including non-numeric ones, which caused
TypeError: agg function failed [how->mean,dtype->object]

Fix

the correct code should explicitly select preTestScore column.
regiment[regiment['regiment'] == 'Nighthawks'] .groupby('regiment')['preTestScore'] .mean()

Output

regiment Nighthawks 15.25 Name: preTestScore, dtype: float64

Compatibility Note

This fix reflects changes introduced in Pandas after 2020:

Older notebooks may still run without error on legacy Pandas.

Modern environments require explicit column selection or numeric_only=True