[bug] change Array.slice() for String.slice()

useage:  Array.slice() should be changed to String.slice()

snippets/extendHex.md

@@ -3,7 +3,7 @@
 Extends a 3-digit color code to a 6-digit color code.
 
 Use `Array.map()`, `split()` and `Array.join()` to join the mapped array for converting a 3-digit RGB notated hexadecimal color-code to the 6-digit form.
-`Array.slice()` is used to remove `#` from string start since it's added once.
+`String.slice()` is used to remove `#` from string start since it's added once.
 ```js
 const extendHex = shortHex =>
   '#' + shortHex.slice(shortHex.startsWith('#') ? 1 : 0).split('').map(x => x+x).join('')