Renamed lesson_code.py to main.py

[Ethan-Arrowood]

Closes #27

Used this python code to do this quickly:

import os
input_dir = 'challenges'
for entry in os.scandir(input_dir):
    if not entry.name.startswith('.') and entry.is_dir():
        print(entry.name)
        sub_dir = input_dir + '/' + entry.name
        for f in os.scandir(sub_dir):
            if f.name == 'lesson_code.py':
                src = sub_dir + '/' + f.name
                dst = sub_dir + '/main.py'
                os.rename(src, dst)