docs: filesystems: vfs: Use correct initial heading

Kernel RST has a preferred heading adornment scheme.  Currently all the
heading adornments follow this scheme except the document heading.

Use correct heading adornment for initial heading.
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/vfs.txt

-
-	      Overview of the Linux Virtual File System
+=========================================
+Overview of the Linux Virtual File System
+=========================================
 
 	Original author: Richard Gooch <rgooch@atnf.csiro.au>