Fix the ios_fullscreen app (#2817)

Fixes the ios_fullscreen add-to-app sample app:

1. The Flutter module was renamed from `flutter_module` to
`flutter_module_fullscreen`. However, the iOS native app had not been
updated to reflect this
2. Flutter's minimum deployment version was raised to 13.0. The iOS
native app targeted iOS 12.4.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [ ] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).


If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md
[changelog]: ../CHANGELOG.md

Co-authored-by: Eric Windmill <eric@ericwindmill.com>

add_to_app/fullscreen/README.md

@@ -5,7 +5,7 @@ Embeds a full screen instance of Flutter into an existing iOS or Android app.
 ## Description
 
 These apps showcase a relatively straightforward integration of
-`flutter_module`:
+`flutter_module_fullscreen`:
 
 * The Flutter module is built along with the app when the app is built.
 * The Flutter engine is warmed up at app launch.
@@ -27,7 +27,7 @@ you're building for both iOS and Android, with both toolchains installed):
   #!/bin/bash
   set -e
 
-  cd flutter_module/
+  cd flutter_module_fullscreen/
   flutter pub get
 
   # For Android builds: