Documentation: PCI: extend subheading underline for "lspci output" section

The underline syntax for "lspci output..." section is off-by-one less
than the section heading's length, hence triggers the warning:

Documentation/PCI/endpoint/pci-vntb-howto.rst:131: WARNING: Title underline too short.

Extend the underline by one to match the heading length.

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>

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

@@ -134,7 +134,7 @@ RootComplex Device
 ==================
 
 lspci Output at Host side
-------------------------
+-------------------------
 
 Note that the devices listed here correspond to the values populated in
 "Creating pci-epf-ntb Device" section above::