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/[email protected]/
Fixes: 0c4b285 ("Documentation: PCI: Add specification for the PCI vNTB function device")
Reported-by: Stephen Rothwell <[email protected]>
Cc: Kishon Vijay Abraham I <[email protected]>
Cc: Lorenzo Pieralisi <[email protected]>
Cc: "Krzysztof Wilczyński" <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Frank Li <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Bagas Sanjaya <[email protected]>
Signed-off-by: Jon Mason <[email protected]>

Author: bagasme

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::