Change the node example in readme to node12
Change the node example in readme to node12

Signed-off-by: Alistair Hey <e9adbcf81c8a36ccc628e09bc92d0e25e1c8e501@heyal.co.uk>

openfaas/faas,alexellis/faas,alexellis/faas,alexellis/faas,alexellis/faas,alexellis/faas,alexellis/faas,openfaas/faas,openfaas/faas,alexellis/faas,alexellis/faas,alexellis/faas