[BUGFIX] Point new user to correct folder on first install

The FIRST_INSTALL needs to be created in the web root of your project
and not in the root of your project as that can be below the web root.

Releases: master
Resolves: #80671
Change-Id: Ib6ffc32e93b18d6792193761970fa9c1c866ba4f
Reviewed-on: https://review.typo3.org/52353


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Thomas Hohn <thomas@hohn.dk>
Reviewed-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>