media: docs: fix references for vimc

As the test drivers got moved, we need to update the
cross-references accordingly.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

MAINTAINERS

@@ -17834,7 +17834,7 @@ L:	linux-media@vger.kernel.org
 S:	Maintained
 W:	https://linuxtv.org
 T:	git git://linuxtv.org/media_tree.git
-F:	drivers/media/platform/vimc/*
+F:	drivers/media/test_drivers/vimc/*
 
 VIRT LIB
 M:	Alex Williamson <alex.williamson@redhat.com>