docs: fix $systemDirectory path in existing project.

[jozefrebjak]

Description

In Adding CodeIgniter4 to an Existing Project docs is an incorrectly defined how to adjust the $systemDirectory property.

$systemDirectory should be defined as in appstarter project.

ROOTPATH constant causes an error with php spark serve like:

PHP Fatal error:  Uncaught Error: Undefined constant "Config\ROOTPATH" in /Users/jrebjak/CodeIgniter4/test-existing-project/spark:72

Stack trace:
#0 {main} thrown in /Users/jrebjak/CodeIgniter4/test-existing-project/spark on line 72

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

Yes!
Config\Paths is instantiated before defining ROOTPATH.

[samsonasik]

Thank you @jozefrebjak