[BUG] Misleading Example provided in LibffmConverter

[tim5go]

The example provided in LibffmConverter is wrong
https://github.com/microsoft/recommenders/blob/27709229cdc4aa7d39ab715789f093a2d21d2661/recommenders/datasets/pandas_df_utils.py#L134-L141

The expected output for last column should be:

field4 
4:6:1 
4:7:1 
4:8:1 
4:9:1
4:10:1  

[miguelgfierro]

thanks @tim5go for reporting this, PR sent