[slim] wrong base in readme

[josefschabasser]

Hi!

The readme is wrong about the slim images: they are based on Debian Stretch.

$ docker run keymetrics/pm2:12-slim cat /etc/debian_version
9.12

keymetrics/pm2:12-slim Dockerfile: https://github.com/keymetrics/docker-pm2/blob/999851207955a4a6ffb2ca50f4e2900b35e2abc0/tags/12/slim/Dockerfile#L1

node:12-slim Dockerfile: https://github.com/nodejs/docker-node/blob/fdd2b251827817ac7bb4f0b2b082483abff8ea77/12/stretch-slim/Dockerfile#L1

Best regards Josef

[josefschabasser]

https://registry.hub.docker.com/_/node/ 12-slim is an alias for 12-stretch-slim