Update isPrime.md

snippets/isPrime.md

@@ -11,7 +11,7 @@ Checks if the provided integer is a prime number.
 ```js
 const isPrime = num => {
   const boundary = Math.floor(Math.sqrt(num));
-  for (var i = 2; i <= boundary; i++) if (num % i === 0) return false;
+  for (let i = 2; i <= boundary; i++) if (num % i === 0) return false;
   return num >= 2;
 };
 ```