These are the next steps outlined in the instructions (which are found here: https://flutter.io/ios-release/):
In Xcode, configure the app version and build:
In Xcode, open Runner.xcworkspace in your app’s ios folder.
Select Product > Scheme > Runner.
Select Product > Destination > Generic iOS Device.
Select Runner in the Xcode project navigator, then select the Runner target in the settings view sidebar.
In the Identity section, update the Version to the user-facing version number you wish to publish.
In the Identity section, update the Build identifier to a unique build number used to track this build on iTunes Connect. Each upload requires a unique build number.
Finally, create a build archive and upload it to iTunes Connect:
Select Product > Archive to produce a build archive.
In the sidebar of the Xcode Organizer window, select your iOS app, then select the build archive you just produced.
Click the Validate… button. If any issues are reported, address them and produce another build. You can reuse the same build ID until you upload an archive.
After the archive has been successfully validated, click Upload to App Store…. You can follow the status of your build in the Activities tab of your app’s details page on iTunes Connect.