[media] Documentation: Update driver's directory in video4linux/fimc.txt

Update the documentation with the driver's path changed in
commit 56fa1a6a
[media] s5p-fimc: Change the driver directory to exynos4-is

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>