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>

Documentation/driver-api/mtd/spi-nor.rst

@@ -34,7 +34,8 @@ Before this framework, the layer is like::
          ------------------------
 	        SPI NOR chip
 
-   After this framework, the layer is like:
+After this framework, the layer is like::
+
                    MTD
          ------------------------
               SPI NOR framework
@@ -45,7 +46,8 @@ Before this framework, the layer is like::
          ------------------------
 	       SPI NOR chip
 
-  With the SPI NOR controller driver (Freescale QuadSPI), it looks like:
+With the SPI NOR controller driver (Freescale QuadSPI), it looks like::
+
                    MTD
          ------------------------
               SPI NOR framework