dummy: documentation is stale

dummy0/1/2 names are always used and there are options to set multiple
dummy devices. Remove the obsolete text

Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=42865


Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>