Update GraphQL Upload docs.

[jghyllebert]

Currently the docs show an outdated example of the upload functionality.

When following the example loosely I get a deprecation warning.

I/flutter (16806): ⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️ DEPRECATION WARNING ⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️
I/flutter (16806):
I/flutter (16806): Please do not use File direcly anymore. Instead, use
I/flutter (16806): MultipartFile. There's also a utitlity method to help you
I/flutter (16806): import 'package:graphql/utilities.dart' show multipartFileFrom;
I/flutter (16806):
I/flutter (16806): ⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️⚠️️️️️️️️ DEPRECA

[micimize]

closed in 4.0.0-beta.1 #648 in migration to gql_http_link