Incorrect instructions in ekaf_demo?

Following https://github.com/helpshift/ekaf/blob/master/src/ekaf_demo.erl#L6 I get:

```
kafkamocker $ rebar compile skip_deps=true && erl -pa ebin -pa deps/*/ebin -s kafkamocker_demo
==> kafkamocker (compile)
Erlang/OTP 17 [erts-6.2] [source] [64-bit] [smp:4:4] [async-threads:10] [kernel-poll:false]

{"init terminating in do_boot",{undef,[{kafkamocker_demo,start,[],[]},{init,start_it,1,[]},{init,start_em,1,[]}]}}

Crash dump was written to: erl_crash.dump
init terminating in do_boot ()
```

Read src/ekaf_demo.erl at the linked line and reproduce the documented rebar compile and erl command. Compare the command with the module and entry point named by the resulting undef error; update the instructions so the documented command starts the demo successfully.

Written by the indexing model from the issue text.