message: Update file exchange example to work with wonka v4
diff: @@ -465,7 +465,7 @@ import { extractFiles } from 'extract-files';
  ...
  const createFetchSource = (operation) => {
  - return make(([next, complete]) => {
  + return make(({next, complete}) => {
  const { context } = operation;
  const { clone, files } = extractFiles(operation.variables);
  const isFileUpload = !!files.size;
  