Fix bug with katakana converter.

src/Classes/Collection.php

@@ -16,7 +16,7 @@ abstract class Collection implements ArrayAccess, JsonSerializable, IteratorAggr
     use Arr;
 
     /**
-     * Collection methods adopted from Laravel Collection.
+     * Collection methods adapted from Laravel Collection.
      * https://github.com/illuminate/support/blob/master/Collection.php.
      */
 

src/Helpers/Arr.php

@@ -7,7 +7,7 @@
 trait Arr
 {
     /**
-     * Methods in this trait adopted from Laravel Arr class.
+     * Methods in this trait adapted from Laravel Arr class.
      * https://github.com/illuminate/support/blob/master/Arr.php
      */
 

src/Helpers/Converter.php

@@ -35,7 +35,7 @@ public static function convert($items, $format)
      *
      * @return string
      */
-    private function katakana($value)
+    private static function katakana($value)
     {
         return static::convertKana($value, 'C');
     }