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/stringPermutations.md

@@ -6,7 +6,7 @@ Generates all permutations of a string (contains duplicates).
 
 Use recursion.
 For each letter in the given string, create all the partial permutations for the rest of its letters.
-Use `Array.map()` to combine the letter with each partial permutation, then `Array.reduce()` to combine all permutations in one array.
+Use `Array.prototype.map()` to combine the letter with each partial permutation, then `Array.prototype.reduce()` to combine all permutations in one array.
 Base cases are for string `length` equal to `2` or `1`.
 
 ```js