net: core: Fix documentation

Fix bad grammar in description of init_dummy_netdev() function.  This
topic showed up in the review of the "allocate dummy device dynamically"
patch set.

Suggested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>