Fix typo in server-add command description.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>
add "flynn kill"

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>
add "flynn server-remove"

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>
add "flynn servers"

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>