Fixed issue on Top 12 German Companies sample

[kikoso]

This PR modifies the handling of the ROA and ROE columns in the 12 German Companies example to address formatting issues. The values were initially strings with periods (e.g., 12.5), which caused incorrect calculations. This PR removes the periods and converts the values to Double to ensure accurate computation of the mean and standard deviation.

[Jolanrensen]

In the cell you modified:

// Group data by sector to compute average and standard deviations of ROA and ROE
val roeAndRoaDf = companiesDf.groupBy { sector }.aggregate {
    ROA.mean() into "Avg ROA"
    ROA.std() into "Std ROA"
    ROE.mean() into "Avg ROE"
    ROE.std() into "Std ROE"
}

roeAndRoaDf

the columns ROA and ROE are both of type Double, not String.
They are Double already from the 4th cell and the line: .convert { ROA and ROE }.with { it.replace(".", "").toDouble() }, so the calculation is done correctly.