MAINTAINERS: r8169: Update path to the driver

Update MAINTAINERS record to reflect the filename change.
The file was moved in commit 25e992a4

 ("r8169: rename
r8169.c to r8169_main.c")

Cc: Heiner Kallweit <hkallweit1@gmail.com>
Cc: nic_swsd@realtek.com
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Denis Efremov <efremov@linux.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>