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: 1e736f1489 ("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>

Documentation/filesystems/proc.rst

@@ -2167,7 +2167,7 @@ where 'size' is the size of the DMA buffer in bytes. 'count' is the file count o
 the DMA buffer file. 'exp_name' is the name of the DMA buffer exporter.
 
 VFIO Device files
-~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~
 
 ::