docs: filesystems: vfs: Use uniform space after period.

Currently sometimes document has a single space after a period and
sometimes it has double.  Whichever we use it should be uniform.

Use double space after period, be uniform.

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