Update content in Dart (#3244)

* Updated 100 Dart Basics 

Dart can also be used to build server and desktop applications.

* Updated 103 Functions

Explanation about dart functions

src/roadmaps/flutter/content/100-dart-basics/103-functions.md

@@ -1,5 +1,7 @@
 # Functions
 
+Dart is a true object-oriented language, so even functions are objects and have a type, Function. This means that functions can be assigned to variables or passed as arguments to other functions. You can also call an instance of a Dart class as if it were a function. 
+
 Visit the following resources to learn more:
 
 - [Functions](https://dart.dev/guides/language/language-tour#functions)

src/roadmaps/flutter/content/100-dart-basics/index.md

@@ -1,6 +1,6 @@
 # Dart Basics
 
-Dart is an open-source, general-purpose, object-oriented programming language with C-style syntax developed by Google in 2011. The purpose of Dart programming is to create a frontend user interfaces for the web and mobile apps.
+Dart is an open-source, general-purpose, object-oriented programming language with C-style syntax developed by Google in 2011. The purpose of Dart programming is to create a frontend user interfaces for the web and mobile apps. It can also be used to build server and desktop applications.
 
 Visit the following resources to learn more: