scripts/documentation-file-ref-check: ignore sched-pelt false positive

When Documentation/scheduler/sched-pelt.c is compiled, it generates
a file called Documentation/scheduler/sched-pelt. As this only
exists after building such tool, we need an explict check
to remove the false-positive.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>