fix error message with new debug syntax

src/rebar3.erl

@@ -55,7 +55,7 @@ main(Args) ->
         Error ->
             %% Nothing should percolate up from rebar_core;
             %% Dump this error to console
-            ?ERROR("Uncaught error in rebar_core. Run with -vvv to see stacktrace~n", []),
+            ?ERROR("Uncaught error in rebar_core. Run with DEBUG=1 to see stacktrace~n", []),
             ?DEBUG("Uncaught error: ~p~n", [Error]),
             rebar_utils:delayed_halt(1)
     end.