Fix Elixir image official repo URL

elixir/github-repo

@@ -1 +1 @@
-https://github.com/c0b/docker-elixir
+https://github.com/erlef/docker-elixir

elixir/maintainer.md

@@ -1 +1 @@
-[the Docker Community](%%GITHUB-REPO%%), [with Elixir's support](https://github.com/docker-library/official-images/pull/1398#issuecomment-180484549)
+[the Erlang Ecosystem Foundation](%%GITHUB-REPO%%)