feat: improve PHPDoc for get() (#232)

Co-authored-by: Christopher Georg <christopher.georg@sr-travel.de>

src/Holidays.php

@@ -35,7 +35,7 @@ public static function has(string $country): bool
         return Country::find($country) !== null;
     }
 
-    /** @return array<array{name: string, date: string}> */
+    /** @return array<array{name: string, date: CarbonImmutable}> */
     public function get(Country|string|null $country = null, ?int $year = null): array
     {
         $country ??= $this->country;