Pad File numbers with leading zeros

There is no leading zero in the files located in python-for-beginners/Slides now.
- 10 - file_name
- 11 - file_name
- 1 - file _name
- 2 - file_name

Add leading zeros to sort like:
- 01 - file_name
- 02 - file_name
- 10 - file_name
- 11 - file_name