generator: rename systemd-mkfs@.service to systemd-makefs@.service

generator: rename systemd-mkfs@.service to systemd-makefs@.service

The second name was used in documentation, and the first in the code that
generated the unit. 'systemd-makefs' is the name we want, for example for
consistency with the systemd-makefs executable.

In principle this breaks compatibility, but in practice this is unlikely to be
noticeable. Each instance of the unit is created by writing out a full
definition, so the template was never defined. So the name could only be used
for ordering, and there is not reason to order things against this unit from
the outside: the ordering would rather be against the final mount unit.

Fixes #11769.