input: Write a function to check if a string is a substring of another string.
  assert isSubstring("Hello", "Hello World") == True
  assert isSubstring("World", "Hello World") == True
  assert isSubstring("Python", "Hello World") == False
  assert isSubstring("lo", "Hello World") == True
  assert isSubstring("world", "Hello World") == False
output: def isSubstring(sub, string):
      return sub in string