README: fix networking example actor spawn
README: fix networking example actor spawn

SpawnTemplate is not defined in the actor package so use FromProducer
instead. This is the most commonly used spawn function in the README so
this change makes it more consistent as well.

AsynkronIT/protoactor-go,AsynkronIT/protoactor-go,AsynkronIT/gam,rogeralsing/gam,AsynkronIT/gam