Change location of @generated

Signed-off-by: Antonín Říha <[email protected]>

Author: anriha

src/betterproto/templates/template.py.j2

@@ -1,6 +1,7 @@
-# @generated by the protocol buffer compiler.  DO NOT EDIT!
+# Generated by the protocol buffer compiler.  DO NOT EDIT!
 # sources: {{ ', '.join(output_file.input_filenames) }}
 # plugin: python-betterproto
+# This file has been @generated
 {% for i in output_file.python_module_imports|sort %}
 import {{ i }}
 {% endfor %}