Avoid confusing prototype methods for static methods

Correct: `Array.from()` (it’s a static method)
Incorrect: `Array.join()` (doesn’t exist; it’s a prototype method)

This patch uses the common `#` syntax to denote `.prototype.`.

snippets/mapKeys.md

@@ -3,7 +3,7 @@
 Creates an object with keys generated by running the provided function for each key and the same values as the provided object.
 
 Use `Object.keys(obj)` to iterate over the object's keys.
-Use `Array.reduce()` to create a new object with the same values and mapped keys using `fn`.
+Use `Array.prototype.reduce()` to create a new object with the same values and mapped keys using `fn`.
 
 ```js
 const mapKeys = (obj, fn) =>