docs: trace: fix the location of kprobes.rst

This patch was moved out of staging.

Fixes: 2165b82f

 ("docs: Move kprobes.rst from staging/ to trace/")

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/a6d4c62e19ab1510789418a3a5ad42980cd7ae3a.1599660067.git.mchehab+huawei@kernel.org


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