[11.x] Fix docblock for \Illuminate\Validation\ClosureValidationRule::message()

[seriquynh]

\Illuminate\Validation\ClosureValidationRule::message() returns an array, not a string.

<?php

namespace Illuminate\Validation;

class ClosureValidationRule
{
   /**
     * The validation error messages.
     *
     * @var array
     */
    public $messages = [];

    /**
     * Get the validation error messages.
     *
     * @return array
     */
    public function message()
    {
        return $this->messages;
    }
}