fix colorcoding

Author: nasko

documentation/20-fields/30-color.md

@@ -35,7 +35,7 @@ print_r( $rgba );
 ```
 
 Output:
-```
+```php
 array(4) {
  ["red"] => int(255), // integer in the 0-255 range
  ["green"] => int(0), // integer in the 0-255 range