Change package and constructor docblock

Closes #3503584

modules/form_decorator_example/form_decorator_example.info.yml

 name: 'Form decorator example'
 type: module
-description: 'Allow to decorate forms'
-package: Custom
+description: 'Provides some example form decorators'
+package: Forms
 core_version_requirement: ^10 || ^11
 dependencies:
   - form_decorator:form_decorator