Use :binary.split for Erlang example

The example here https://github.com/devonestes/fast-elixir/blob/master/code/general/string_split_large_strings.exs#L21 uses Erlang's string module but this is not a fair comparison since Elixir strings are binaries. Maybe this should have a test with https://www.erlang.org/doc/man/binary.html#split-3:

```erlang
binary:split(Str, <<",">>, [global]).
```