spi: docs: spidev: fix echo command format

The two example echo commands for binding the spidev driver were being
rendered as one line in the HTML output. This patch makes use of the
restructured text :: to format the commands as a code block instead
which preserves the line break.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://msgid.link/r/20240319183344.2106335-1-dlechner@baylibre.com


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