docs: fix references for ipmi.rst file

As this file got moved, fix references for it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/b6c1ded590f27198de15b16237509128e55fa810.1588345503.git.mchehab+huawei@kernel.org


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