linux-next: SLIMbus: doc: Fix a warning "Title underline too short"

This patch fixes a warning during "make xmldocs"

Documentation/driver-api/slimbus.rst:93:
WARNING: Title underline too short.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>