use 20 as example

[getong]

20 is released, and I use 20 as example.
There are something more to be changed, I think you can change it. @c0b

[c0b]

This is good; although we don't usually revise the README, because version numbers update too often. But generally revise it once or twice a year should be good; and the docker-library/docs's erlang portion also need to be revised; it serves as docker hub entry page https://hub.docker.com/_/erlang/

1. https://github.com/docker-library/docs/tree/master/erlang
2. https://github.com/docker-library/docs/blob/master/erlang/content.md (this is the only one need changes, the above front README.md is auto-generated)

also need onbuild examples from #47

[getong]

@c0b I think we can change it step by step. And for now, what can I do?
docker-library/docs#947 is made.