doc: net: dsa: Fix typo in config code sample

In the "single port" example code for configuring a DSA switch without
tagging support from userspace the command to bring up the "lan2" link
was typo'd.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>