esl: fix package name; fix recipe name in README

Erlang Solutions seems to be using "esl-erlang" for all packages on all platforms. The esl recipe was broken for me since it was trying to install "erlang" for RedHat systems.

I've changed this and I've also changed the README to refer to the esl recipe by its current name.

I have executed a CLA before on my own behalf, but since this change is a direct result of work I'm doing for my employer (Sleepy Giant), I'm not sure if I need to have them do a CCLA or if they already have one (I haven't asked yet).