docs: trace: ftrace.rst: avoid using ReST :doc:`foo` markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/cf9b03ff4b7917d9846503f198372bc6b821445b.1623824363.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>