Update fibonacci.js

I made the exchange of value shorter.

fibonacci/fibonacci.js

@@ -4,9 +4,7 @@ const fibonacci = function(count) {
   let a = 0;
   let b = 1;
   for (let i = 1; i < count; i++) {
-    const temp = b;
-    b = a + b;
-    a = temp;
+    [a, b] = [b, a+b];
   }
   return b;
 };