Documentation/fb/vesafb.txt: "inverse" option corrections

Spell the vesafb "inverse" option correctly and tell what it does.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-fbdev@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Antonino A. Daplas <adaplas@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>