docs: filesystems: vfs: Use uniform spacing around headings

Currently spacing before and after headings is non-uniform.  Use two
blank lines before a heading and one after the heading.

Use uniform spacing around headings.

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