docs: not union but atomic

Author: kenjis

user_guide_src/source/changelogs/v4.3.0.rst

@@ -52,7 +52,7 @@ Changes
     - The ``CodeIgniter\CLI\CommandRunner`` class has been removed due to a change in Spark commands processing.
     - The system route configuration file ``system/Config/Routes.php`` has been removed.
     - The route configuration file ``app/Config/Routes.php`` has been changed. Removed include of system routes configuration file.
-- All union type properties in ``Config`` classes have been typed.
+- All atomic type properties in ``Config`` classes have been typed.
 
 Deprecations
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