add api key to get request flutter code example

Example: how to use api key in flutter

Try to change http.Response response = await http.get(cryptoUrl); to

http.Response response = await http.get(cryptoUrl, 
headers: {"X-CMC_PRO_API_KEY": "cab79c7b-52e9-4e4b-94fc-b0f32da14799"});

For more info check this link.