Clarify user message in phx.gen.json

[ponychicken]

The statement "Add the resource to your :api scope" is confusing for new-comers.

Users will open the router.ex file, and quickly find a section:

  pipeline :api do
    plug :accepts, ["json"]
  end

and paste the line into this block.

Experienced phoenix devs will understand that this is a pipeline and not a scope, but these semantics will not be obvious the newby.

The pull request changes the line to read:
"Add the resource to "/api" scope" or ( "Add the resource to "/api/v1" scope" if you have configured the api_endpoint like this)

I was struggling with this, and this user on the forums as well:
https://elixirforum.com/t/can-someone-please-help-me-with-this-basic-api-creation-rubber-ducky-edition/53684/7

[josevalim]

Thank you for the PR! Can you please update the failing tests?

[ponychicken]

Tests are working locally now

[josevalim]

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