media: correct MEDIA_TEST_SUPPORT help text

Fix grammar/wording in the help text for MEDIA_TEST_SUPPORT.

Fixes: 4b32216a ("media: split test drivers from platform directory")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>