media: dvb/intro.rst: Use verbatim font where needed

The device numbering for DVB uses "M" and "N" as vars for the
number of the device, but sometimes this is printed using normal
font instead of verbatim.

While here, remove an extra space after quotation marks.

This is a minor cleanup with no changes at the text.

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