docs: driver-api: mtd: intel-spi: Improve formatting of shell commands

Format the shell commands and output in steps 6 and 8 as code blocks,
for consistency with the rest of the document.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20201108151005.1843666-1-j.neuschaefer@gmx.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>