Clarify user message in phx.gen.json

guides/mix_tasks.md

@@ -154,7 +154,7 @@ $ mix phx.gen.json Blog Post posts title:string content:string
 When `mix phx.gen.json` is done creating files, it helpfully tells us that we need to add a line to our router file as well as run our Ecto migrations.
 
 ```console
-Add the resource to your :api scope in lib/hello_web/router.ex:
+Add the resource to the "/api" scope in lib/hello_web/router.ex:
 
     resources "/posts", PostController, except: [:new, :edit]
 

lib/mix/tasks/phx.gen.json.ex

@@ -171,7 +171,7 @@ defmodule Mix.Tasks.Phx.Gen.Json do
     else
       Mix.shell().info("""
 
-      Add the resource to your :api scope in #{Mix.Phoenix.web_path(ctx_app)}/router.ex:
+      Add the resource to the "#{Application.get_env(ctx_app, :generators)[:api_prefix] || "/api"}" scope in #{Mix.Phoenix.web_path(ctx_app)}/router.ex:
 
           resources "/#{schema.plural}", #{inspect(schema.alias)}Controller, except: [:new, :edit]
       """)

test/mix/tasks/phx.gen.json_test.exs

@@ -113,7 +113,7 @@ defmodule Mix.Tasks.Phx.Gen.JsonTest do
                       [
                         """
 
-                        Add the resource to your :api scope in lib/phoenix_web/router.ex:
+                        Add the resource to the "/api" scope in lib/phoenix_web/router.ex:
 
                             resources "/posts", PostController, except: [:new, :edit]
                         """