add explanation to mandatory-errors/1

Author: XiaoQuark

Sprint-1/2-mandatory-errors/1.js

@@ -2,3 +2,7 @@
 
 let age = 33;
 age = age + 1;
+
+// const are variables which value cannot change
+// let is a variable which value can be updated
+// to fix the error I modified the variable declaration and used let instead of const