bug in preprocessing.py

[zyf505]

Lines 23-25: the mapping isn't created correctly, key and value are in reversed order.
Should be something like

            mappings[col] = {v: i for i, v in 
                enumerate(df_train[col].astype("category").cat.categories)
            }

[Sathya Kamesh]

Fixed this bug! Thank you for pointing it out!