Fix new atomvm_app name in rebar.config

Fixes the rebar.config template file for new applications created with
`rebar3 atomvm new atomvm_app ${{name}}` to insert the newly created application
name in the rebar.config file for the appliction, rather than always using
`myapp` for the application name.

Signed-off-by: Winford <winford@object.stream>

Author: UncleGrumpy

priv/rebar.config

@@ -21,5 +21,5 @@
     atomvm_rebar3_plugin
 ]}.
 {atomvm_rebar3_plugin, [
-    {packbeam, [{start, myapp}, prune]}
+    {packbeam, [{start, {{name}}}, prune]}
 ]}.