Solution mutate-an-array-declared-with-const.english.md (#18759)

* Update mutate-an-array-declared-with-const.english.md

* Improved JavaScript style

curriculum/challenges/english/02-javascript-algorithms-and-data-structures/es6/mutate-an-array-declared-with-const.english.md

@@ -44,7 +44,7 @@ tests:
 ```js
 const s = [5, 7, 2];
 function editInPlace() {
-  "use strict";
+  'use strict';
   // change code below this line
 
   // s = [2, 5, 7]; <- this is invalid
@@ -64,6 +64,17 @@ editInPlace();
 <section id='solution'>
 
 ```js
-// solution required
+const s = [5, 7, 2];
+function editInPlace() {
+  'use strict';
+  // change code below this line
+
+  // s = [2, 5, 7]; <- this is invalid
+  s[0] = 2;
+  s[1] = 5;
+  s[2] = 7;
+  // change code above this line
+}
+editInPlace();
 ```
 </section>