docs: mtd: spi-nor: Fix formatting of text vs. diagrams

Currently, all diagrams below "Before this framework, the layer is like"
and the text in between are rendered as one monospace text block.
Instead, each individual diagram should be a monospace text block.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20201107183902.1182809-1-j.neuschaefer@gmx.net


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