Solution prevent-object-mutation.english.md (#18767)

* Update prevent-object-mutation.english.md

* Improved JavaScript style

curriculum/challenges/english/02-javascript-algorithms-and-data-structures/es6/prevent-object-mutation.english.md

@@ -41,7 +41,7 @@ tests:
 
 ```js
 function freezeObj() {
-  "use strict";
+  'use strict';
   const MATH_CONSTANTS = {
     PI: 3.14
   };
@@ -69,6 +69,22 @@ const PI = freezeObj();
 <section id='solution'>
 
 ```js
-// solution required
+function freezeObj() {
+  'use strict';
+  const MATH_CONSTANTS = {
+    PI: 3.14
+  };
+  // change code below this line
+  Object.freeze(MATH_CONSTANTS);
+
+  // change code above this line
+  try {
+    MATH_CONSTANTS.PI = 99;
+  } catch( ex ) {
+    console.log(ex);
+  }
+  return MATH_CONSTANTS.PI;
+}
+const PI = freezeObj();
 ```
 </section>