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>