message: docs: change the file name of the public API according with Angular CLI
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diff: @@ -42,7 +42,7 @@ This builder, among other things, ensures that the library is always built with
  To make library code reusable you must define a public API for it. This "user layer" defines what is available to consumers of your library. A user of your library should be able to access public functionality (such as NgModules, service providers and general utility functions) through a single import path.
  -The public API for your library is maintained in the `index.ts` file of your library folder.
  +The public API for your library is maintained in the `public-api.ts` file in your library folder.
  Anything exported from this file is made public when your library is imported into an application.
  Use an NgModule to expose services and components.
  