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/isUpperCase.md

@@ -2,7 +2,7 @@
 
 Checks if a string is upper case.
 
-Convert the given string to upper case, using `String.toUpperCase()` and compare it to the original.
+Convert the given string to upper case, using `String.prototype.toUpperCase()` and compare it to the original.
 
 
 ```js