docs: $systemDirectory path in existing project.

Author: jozefrebjak

user_guide_src/source/installation/installing_composer.rst

@@ -152,7 +152,7 @@ Setting Up
 
     1. Copy the **app**, **public**, **tests** and **writable** folders from **vendor/codeigniter4/framework** to your project root
     2. Copy the **env**, **phpunit.xml.dist** and **spark** files, from **vendor/codeigniter4/framework** to your project root
-    3. You will have to adjust the ``$systemDirectory`` property in **app/Config/Paths.php** to refer to the vendor one, e.g., ``ROOTPATH . '/vendor/codeigniter4/framework/system'``.
+    3. You will have to adjust the ``$systemDirectory`` property in **app/Config/Paths.php** to refer to the vendor one, e.g., ``__DIR__ . '/../../vendor/codeigniter4/framework/system'``.
 
 Initial Configuration
 ---------------------