[PHP 8.2] Fix ${var} string interpolation deprecation (#142)

PHP 8.2 deprecates `"${var}"` string interpolation pattern.
This fixes all three of such occurrences in `dragonmantank/cron-expression` package.

 - [PHP 8.2: `${var}` string interpolation deprecated](https://php.watch/versions/8.2/${var}-string-interpolation-deprecated)
 - [RFC](https://wiki.php.net/rfc/deprecate_dollar_brace_string_interpolation)

src/Cron/DayOfMonthField.php

@@ -49,7 +49,7 @@ class DayOfMonthField extends AbstractField
     private static function getNearestWeekday(int $currentYear, int $currentMonth, int $targetDay): ?DateTime
     {
         $tday = str_pad((string) $targetDay, 2, '0', STR_PAD_LEFT);
-        $target = DateTime::createFromFormat('Y-m-d', "${currentYear}-${currentMonth}-${tday}");
+        $target = DateTime::createFromFormat('Y-m-d', "{$currentYear}-{$currentMonth}-{$tday}");
 
         if ($target === false) {
             return null;