Update weightedAverage (#1251)

Co-authored-by: Angelos Chalaris <chalarangelo@gmail.com>

snippets/weightedAverage.md

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+---
+title: weightedAverage
+tags: math,array,beginner
+---
+
+Returns the weighted average of two or more numbers.
+
+- Use `Array.prototype.reduce()` to add each value to an accumulator, initialized with a value of `0` and divide by the `length` of the array.
+
+```js
+const weightedAverage = (nums, weights) => 
+  nums.reduce((acc, n, i) => acc + (weights[i] * n), 0) / nums.length;
+```
+
+```js
+weightedAverage([1, 2, 3], [0.6,0.2,0.3]); // 0.6333
+```