Documentation: PCI: Use code-block block for scratchpad registers diagram

The diagram in "Scratchpad Registers" isn't formatted inside code block,
hence triggers indentation warning:

Documentation/PCI/endpoint/pci-vntb-function.rst:82: WARNING: Unexpected indentation.

Fix the warning by using code-block directive to format the diagram
inside code block, as in other diagrams in Documentation/. While at it,
unindent the preceeding text.

Link: https://lore.kernel.org/linux-next/[e-mail]/
Fixes: 0c4b285d9636cc ("Documentation: PCI: Add specification for the PCI vNTB function device")
Reported-by: Stephen Rothwell <[e-mail]>
Cc: Kishon Vijay Abraham I <[e-mail]>
Cc: Lorenzo Pieralisi <[e-mail]>
Cc: "Krzysztof Wilczyński" <[e-mail]>
Cc: Bjorn Helgaas <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Frank Li <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Jon Mason <[e-mail]>

Documentation/PCI/endpoint/pci-vntb-function.rst

···
 Scratchpad Registers:
 ---------------------
 
-  It is appended after Config region.
+It is appended after Config region.
+
+.. code-block:: text
+
 
   +--------------------------------------------------+ Base
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