Fix phx.gen.socket moduledoc grammar and example path

[03juan]

Hi there.

This fixes some small grammar inconsistencies, as well as the path for the umbrella app example.

[03juan]

~/code/elixir/tests/my_app_umbrella                 13:44:52
❯ cd apps/my_app_web 

~/code/elixir/tests/my_app_umbrella/apps/my_app_web
❯ mix phx.gen.socket User
* creating lib/my_app_web/channels/user_socket.ex
* creating assets/js/user_socket.js

Add the socket handler to your `lib/my_app_web/endpoint.ex`, for example:

    socket "/socket", MyAppWeb.UserSocket,
      websocket: true,
      longpoll: false

For the front-end integration, you need to import the `user_socket.js`
in your `assets/js/app.js` file:

    import "./user_socket.js"

[chrismccord]

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