Update README.md (#95)

I tried copying the sample code as it is but did not do what I assumed.
Specifically, it is as follows.

- It does not work without `require_once`
- I can not check the output if it is `return`

I modified the README.md.
I think that this one is better.
Because you can use it as soon as you copy it.

README.md

@@ -16,6 +16,8 @@ PHP-ML requires PHP >= 7.0.
 
 Simple example of classification:
 ```php
+require_once 'vendor/autoload.php';
+
 use Phpml\Classification\KNearestNeighbors;
 
 $samples = [[1, 3], [1, 4], [2, 4], [3, 1], [4, 1], [4, 2]];
@@ -24,7 +26,7 @@ $labels = ['a', 'a', 'a', 'b', 'b', 'b'];
 $classifier = new KNearestNeighbors();
 $classifier->train($samples, $labels);
 
-$classifier->predict([3, 2]);
+echo $classifier->predict([3, 2]);
 // return 'b'
 ```