fix(man): add missing initrd-root-device.target to flow chart

[aafeijoo-suse]

This target was added since https://github.com/dracutdevs/dracut/commit/d4efc0ae and is ordered between basic.target and sysroot.mount, see man bootup.7 (dracut-pre-mount.service starts after).

Checklist

• [X] I have tested it locally
• [X] I have reviewed and updated any documentation if relevant
• [ ] I am providing new code and test(s) for it