Correct NGramFrequency class documentation typo

Fixed a minor typo in the docstring of the NGramFrequency class. The change ensures the documentation accurately reflects the function's behavior and improves its readability.

src/NGramFrequency.php

@@ -32,7 +32,7 @@ public static function trigram(array $tokens): array
     }
 
     /**
-     * Generates bigrams with frequency for given array of tokens and n-gram level.
+     * Generates bigrams with frequency for given an array of tokens and n-gram level.
      *
      * @param  array<string>  $tokens
      * @return array<string, float>