Fixed rebar.config shell_apps setting

rebar.config.sample

@@ -211,7 +211,7 @@
 
 %% apps to auto-boot with `rebar3 shell'; defaults to apps
 %% specified in a `relx' tuple, if any.
-{shell_apps, [app1, app2]}
+{shell, [{apps, [app1, app2]}]}.
 
 %% == xref ==