spi: spi-summary.rst: fix underline length

The change to use "target" requires an underline to be extended by
one more character to fix a documentation build warning:

  Documentation/spi/spi-summary.rst:274: WARNING: Title underline too short.
  Declare target Devices
  ^^^^^^^^^^^^^^^^^^^^^

Fixes: hash ("spi: Update the "master/slave" terminology in documentation")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Dhruva Gole <d-gole@ti.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: linux-spi@vger.kernel.org
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Link: https://msgid.link/r/20240216051637.10920-1-rdunlap@infradead.org


Signed-off-by: Mark Brown <broonie@kernel.org>