media: Documentation: media: dtv-frontend.rst: fix a few minor typos

Fix a few minor typos throughout the document without changing the
meaning of the sentences.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>