aaudio: Update aaudio_loopback usage screen

-- replace 'loopback' with 'aaudio_loopback', since this is how
   the executable is called;

-- remove "-f" argument, as it's not implemented;

-- in the example, replace "-b2" to "-n2" ("-b2" doesn't seem correct);

-- brush up text layout.

Change-Id: Ifae87380d0c9378f936f661ad77cb94a944c05fc
Test: make and run