Update the example on Rosetta Code

[oubiwann]

Here: https://rosettacode.org/wiki/Arithmetic/Complex#LFE

Tasks:

• Update to build with the new rebar3_lfe plugin
• Update the tests to use that too
• Update docs (if needed)
• Edit the Rosetta example to show:
  • Note to install Erlang and Rebar
  • Note to clone the repo