Make median.md immutable

`.sort()` mutates the original array

snippets/median.md

@@ -7,7 +7,7 @@ Return the number at the midpoint if `length` is odd, otherwise the average of t
 
 ```js
 const median = arr => {
-  const mid = Math.floor(arr.length / 2), nums = arr.sort((a, b) => a - b);
+  const mid = Math.floor(arr.length / 2), nums = [...arr].sort((a, b) => a - b);
   return arr.length % 2 !== 0 ? nums[mid] : (nums[mid - 1] + nums[mid]) / 2;
 };
 // median([5,6,50,1,-5]) -> 5