[BUGFIX] Properly reference config.yaml instead of config.yml

There is one wrong documentation about config.yaml of
site configuration, where it is `config.yml`, however,
TYPO3 Core always uses the `yaml` file ending.

Resolves: #86752
Releases: master
Change-Id: I06605ca83baa453bc64da96d321f7a22b0893687
Reviewed-on: https://review.typo3.org/58709


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>