Update Feb-18-2020.md

Author: mariampogosyan

Feb-18-2020.md

@@ -3,7 +3,7 @@
 ## Easy
 ### [Employee Importance](https://github.com/WomenWhoCodeNYC/Algorithms/blob/master/challenges/employeeImportance/employeeImportance.md)
 ### [Palindrome](https://github.com/WomenWhoCodeNYC/Algorithms/blob/master/challenges/palindrome/palindrome.md)
-### [Add Strings](https://github.com/WomenWhoCodeNYC/Algorithms/blob/master/challenges/addStrings/addStrings.md)
+### [Missing Number](https://github.com/WomenWhoCodeNYC/Algorithms/blob/master/challenges/missingNumber/missingNumber.md)
 
 ## Medium
 ### [Similar RGB Color](https://github.com/WomenWhoCodeNYC/Algorithms/blob/master/challenges/grumpyBookstoreOwner/grumpyBookstoreOwner.md)