[DOCS] Improve documentation for the dedicated log file example

[lukas-maxheim]

The change adapts the code sample for setting up a dedicated log file for LDAP:

• https://docs.typo3.org/p/causal/ig_ldap_sso_auth/main/en-us/AdministratorManual/Logging.html#dedicated-log-file-for-ldap

The standard TYPO3 namespace "\TYPO3\CMS\Core\Core\" is missing which can cause several issues.