Refactor modify method return type to use self instead of DateTime|false (#129)

Author: EthannSchneider

lib/DateTime.php

@@ -201,7 +201,7 @@ public function setTimezone(\DateTimeZone $timezone): DateTime
         return parent::setTimezone($timezone);
     }
 
-    public function modify(string $modify = ''): DateTime|false
+    public function modify(string $modify = ''): self
     {
         $this->flag_dirty();