docs: proc.rst: Fix VFIO Device title formatting

docs: proc.rst: Fix VFIO Device title formatting

Title underline is one character too short.

Cc: Alex Mastro <amastro@fb.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20250828123035.2f0c74e7@canb.auug.org.au
Fixes: 1e736f148956 ("vfio/pci: print vfio-device syspath to fdinfo")
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20250828203629.283418-1-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>