input: Write a function to replace blank spaces with any character in a string.
  assert replace_blank("hello people",'@')==("hello@people")
  assert replace_blank("python program language",'$')==("python$program$language")
  assert replace_blank("blank space","-")==("blank-space")
  
output: def replace_blank(s, char):
      return s.replace(' ', char)