docs: blockdev: change title to match section content

This index.rst was added in commit
39443104c7d3 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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/admin-guide/blockdev/index.rst

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