Added resourses in courses for flutter (#7709)

* Added resourses in courses for flutter

* Corrected a new line error

courses/free-courses-en.md

@@ -448,10 +448,12 @@
 
 ### Flutter
 
+* [Flutter & Firebase App Build](https://www.youtube.com/playlist?list=PL4cUxeGkcC9j--TKIdkb3ISfRbJeJYQwC) - The Net Ninja
 * [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)
 * [Flutter Tutorial for Beginners](https://www.udemy.com/course/learn-flutter-beginners-course) - Mayuresh Wankhede (Udemy)
+* [Flutter Tutorial for Beginners](https://www.youtube.com/playlist?list=PL4cUxeGkcC9jLYyp2Aoh6hcWuxFDX6PBJ) - The Net Ninja
 * [Free Flutter Course](https://www.youtube.com/playlist?list=PL6yRaaP0WPkVtoeNIGqILtRAgd3h2CNpT) - Vandad Nahavandipoor (YouTube)