spi: Update reference to struct spi_controller

struct spi_master has been renamed to struct spi_controller. Update the
reference in spi.rst to make it clickable again.

Fixes: 8caab75f

 ("spi: Generalize SPI "master" to "controller"")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20221101173252.1069294-1-j.neuschaefer@gmx.net
Signed-off-by: Mark Brown <broonie@kernel.org>