[Elixir] Improve `language-list` instructions code snippets

[angelikatyborska]

I noticed that the function names in the last two steps were incorrect, so I'm fixing that. I also split each function call into its own line so that it's easier to read on the website on a narrower screen, and I extended the code snippets a bit to better show that the languages are prepended to the list, not appended.