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/20220621200235.211b2e32@canb.auug.org.au/


Fixes: 0c4b285d9636cc ("Documentation: PCI: Add specification for the PCI vNTB function device")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Lorenzo Pieralisi <lpieralisi@kernel.org>
Cc: "Krzysztof Wilczyński" <kw@linux.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Frank Li <Frank.Li@nxp.com>
Cc: linux-pci@vger.kernel.org
Cc: linux-next@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>