Fix typo

Fixed `archieved => achieved`

README.md

@@ -313,7 +313,7 @@ Comparison:
 
 ##### `Array#length` vs `Array#size` vs `Array#count` [code](code/array/length-vs-size-vs-count.rb)
 
-Use `#length` when you only want to know how many elements in the array, `#count` could also archieve this. However `#count` should be use for counting specific elements in array. [Note `#size` is an alias of `#length`](https://github.com/ruby/ruby/blob/f8fb526ad9e9f31453bffbc908b6a986736e21a7/array.c#L5817-L5818).
+Use `#length` when you only want to know how many elements in the array, `#count` could also achieve this. However `#count` should be use for counting specific elements in array. [Note `#size` is an alias of `#length`](https://github.com/ruby/ruby/blob/f8fb526ad9e9f31453bffbc908b6a986736e21a7/array.c#L5817-L5818).
 
 ```
 $ ruby -v code/array/length-vs-size-vs-count.rb