input: Write a python function to check if a substring exists within a string.
  assert check_substring("Hello world!", "world") == True
  assert check_substring("OpenAI ChatGPT", "GPT") == True
  assert check_substring("Python programming", "Java") == False
  assert check_substring("Data Science", "Science") == True
  assert check_substring("Artificial Intelligence", "Intelligent") == False
output: def check_substring(main_string, substring):
      return substring in main_string