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>

Documentation/media/uapi/dvb/intro.rst

@@ -137,9 +137,9 @@ individual devices are called:
 
 -  ``/dev/dvb/adapterN/caM``,
 
-where N enumerates the DVB PCI cards in a system starting from 0, and M
+where ``N`` enumerates the DVB PCI cards in a system starting from 0, and ``M``
 enumerates the devices of each type within each adapter, starting
-from 0, too. We will omit the “ ``/dev/dvb/adapterN/``\ ” in the further
+from 0, too. We will omit the “``/dev/dvb/adapterN/``\ ” in the further
 discussion of these devices.
 
 More details about the data structures and function calls of all the