updating the README with MissingNumber.dart

Author: Farial-mahmod

README.md

@@ -2,7 +2,7 @@
 ## Using Dart to solve Leetcode problems (Continuously updated)
 
 ### Why Dart:
-Most problem solutions are found either in Java or in C / C++ on the internet . Dart is an emerging and promising lanaguage . It has most of the features to solve programming problems - if not all and it is in developemnt adding amazing features . It's powering all the Flutter apps . Solving Leetcode problems in Dart would help Fluter and Dart newcomers to grasp Dart quickly and enable to use Dart both in developing apps and solving competitive programming problems without the need to switch to another language . 
+Most problem solutions are found either in Java or in C / C++ on the internet. Dart is an emerging and promising lanaguage. It has most of the features to solve programming problems - if not all and it is in developemnt adding amazing features. It's powering all the Flutter apps. Solving Leetcode problems in Dart would help Fluter and Dart newcomers to grasp Dart quickly and enable to use Dart both in developing apps and solving competitive programming problems without the need to switch to another language. 
 
 ### Solutions with corresponding file names:
 
@@ -15,7 +15,7 @@ Most problem solutions are found either in Java or in C / C++ on the internet .
 | Base 7                  |  BaseSeven.dart             |
 | Contains Duplicate      |  ContainsDuplicate.dart     |
 | Move Zeroes             |  MoveZeroes.dart            |
-
+| Missing Number          |  MissingNumber.dart         |