bug fix best_pred_col

[nielsvanetten]

When for example pred_target_janet_v4_20 is the best_pred_col variable. Than its unfindable in the validation_data at: validation_data["prediction"] = validation_data[best_pred_col].rank(pct=True). This is because here the column is defined as: pred_target_janet_v4_20_downsample(x).

Here is a fix.