Added freeCodeCamp's Flutter Course for Beginners (#8488)

* Update free-courses-en.md

* Update free-courses-en.md

* Update free-courses-en.md

* Update free-courses-en.md

* Update free-courses-en.md

courses/free-courses-en.md

@@ -504,6 +504,7 @@
 ### Flutter
 
 * [Flutter & Firebase App Build](https://www.youtube.com/playlist?list=PL4cUxeGkcC9j--TKIdkb3ISfRbJeJYQwC) - The Net Ninja
+* [Flutter Course for Beginners - 37 hour](https://www.youtube.com/watch?v=VPvVD8t02U8) - freeCodeCamp.org
 * [Flutter Crash Course](https://fluttercrashcourse.com/courses/basics/lessons/materialapp-scaffold-appbar-text) - Nick Manning *(account required)*
 * [Flutter State Management Course](https://www.youtube.com/playlist?list=PL6yRaaP0WPkUf-ff1OX99DVSL1cynLHxO) - Vandad Nahavandipoor (YouTube)
 * [Flutter Tutorial](https://www.udacity.com/course/build-native-mobile-apps-with-flutter--ud905) - Matt Sullivan, James Williams, Mary Xia (Udacity)