docs: blockdev: change title to match section content

This index.rst was added in commit
39443104 docs: blockdev: convert to ReST

It appears that the title from the RapidIO index page was copied. This
title does not match the content of this directory. Change it to match.
Signed-off-by: Joel Colledge <joel.colledge@linbit.com>
Link: https://lore.kernel.org/r/20220530142849.717-1-joel.colledge@linbit.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/blockdev/index.rst

 .. SPDX-License-Identifier: GPL-2.0
 
-===========================
-The Linux RapidIO Subsystem
-===========================
+=============
+Block Devices
+=============
 
 .. toctree::
    :maxdepth: 1