input: Write a python function to fetch the current stock price of a given company using an API call.
  assert fetch_stock_price("Apple") is not None
  assert fetch_stock_price("Google") is not None
  assert isinstance(fetch_stock_price("Amazon"), str)
output: import requests
  
  def fetch_stock_price(company_name):
      api_url = f"https://api.example.com/stock/{company_name}"
      response = requests.get(api_url)
      if response.status_code == 200:
          data = response.json()
          return str(data['price'])
      else:
          return None