input: Write a function to replace a given substring in a string with another string. The function should take three parameters: the original string, the substring to be replaced, and the new substring that will replace it. It should return the modified string.
  assert replace_substring("I love walking on the beach", "walking", "swimming") == "I love swimming on the beach"
  assert replace_substring("Python is great", "Python", "Java") == "Java is great"
  assert replace_substring("Programming is fun", "fun", "interesting") == "Programming is interesting"
  assert replace_substring("Hello World", "Hello", "Goodbye") == "Goodbye World"
  assert replace_substring("No replacements needed", "needed", "needed") == "No replacements needed"
output: def replace_substring(original_string, old_substring, new_substring):
      return original_string.replace(old_substring, new_substring)