README.md: the run.py default for OSv Guest requested RAM size is 2GB RAM, not 1GB

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>