This patch fixes a warning during "make xmldocs"

Documentation/driver-api/slimbus.rst:93:
WARNING: Title underline too short.
Signed-off-by: NMasanari Iida <standby24x7@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

SLIMbus (Serial Low Power Interchip Media Bus) is a specification
developed by MIPI (Mobile Industry Processor Interface) alliance.
SLIMbus is a 2-wire implementation, which is used to communicate with
peripheral components like audio-codec.

The summary of SLIMbus and API is documented in the 'summary' file.
Signed-off-by: NSagar Dharia <sdharia@codeaurora.org>
Signed-off-by: NSrinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviwed-by: NMark Brown <broonie@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>