tests/eas/preliminary: Fix wording in docstring
tests/eas/preliminary: Fix wording in docstring

"Root domain" is confusing, it sounds like it refers to the root_domain
structure in the scheduler. This was in fact meant to refer to the
highest (i.e. in practice 'DIE' level) sched_domain.

ARM-software/lisa,credp/lisa,ARM-software/lisa,arnoldlu/lisa,ARM-software/lisa,credp/lisa,credp/lisa,mdigiorgio/lisa,mdigiorgio/lisa,bjackman/lisa,bjackman/lisa,arnoldlu/lisa,ARM-software/lisa,credp/lisa