fix dracut config in root on ZFS docs

[zeromind]

step 3 incorrectly uses "forced_drivers"; the dracut config option is "force_drivers" (force, not forced)

see https://github.com/search?q=repo%3Adracutdevs%2Fdracut+force_drivers&type=code

[ghost]

Thanks and applied onto PR https://github.com/openzfs/openzfs-docs/pull/423

You can view the upcoming, simplified guide here:

https://github.com/ne9z/openzfs-docs/blob/master/docs/Getting%20Started/RHEL-based%20distro/Root%20on%20ZFS.rst

Makes me wonder why dracut does not emit any warning or errors about a invalid config.

[gmelikov]

Thank you, merged in #423