dt-bindings: net: dsa: Fix typo

Fix spelling mistake documenation -> documentation.

Fixes: 5a18bb14 ("dt-bindings: net: dsa: Let dsa.txt refer to dsa.yaml")
Signed-off-by: Kurt Kanzenbach <kurt@kmk-computers.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>