khepri_machine: Remove @private tag for make_virgin_state/1

[Why]
Apparently EDoc doesn't like this directive for code behind a
`-ifdef(TEST).` macro. It fails with the following error:

    .../src/khepri_machine.erl, in module footer: at line 1994: tag @Private not allowed here.
    edoc: skipping source file '.../src/khepri_machine.erl': {'EXIT',error}.

[How]
Just delete the tag for now. The function is not exported anyway when
the doc is built.

src/khepri_machine.erl

@@ -1991,7 +1991,6 @@ set_metrics(#khepri_machine{} = State, Metrics) ->
 -spec make_virgin_state(Config) -> State when
       Config :: khepri_machine:machine_config(),
       State :: khepri_machine:state().
-%% @private
 
 make_virgin_state(Config) ->
     #khepri_machine{config = Config}.