Documentation/fb: corrections for fbcon.txt

Fix grammar, spacing, indentation, and Kconfig menu locations
in fbcon.txt.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Antonino Daplas <adaplas@pol.net>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>