media: dvb/intro: use the term Digital TV to refer to the system

On several places at the introduction, a digital TV board and its
kernel support is called as DVB. The reason is simple: by the
time the document was written, there were no other digital TV
standards :-)

Modernize the specs by referring to them as Digital TV.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>