input: Write a python function to format a dictionary into a JSON string with proper indentation.
  assert format_dictionary({"name": "John", "age": 30, "city": "New York"}) == '{\n  "name":"John",\n  "age":30,\n  "city":"New York"\n}'
  assert format_dictionary({"name": "Jane", "age": 25, "city": "Los Angeles"}) == '{\n  "name":"Jane",\n  "age":25,\n  "city":"Los Angeles"\n}'
output: import json
  
  def format_dictionary(d):
      return json.dumps(d, indent=2, separators=(',', ':'))