Fix outdated flash component example in docs (#6631)

The example used `show("#flash")` which does not match the default
generated id (`flash-#{kind}`), and was missing the `hidden` attribute
needed for `phx-mounted` to work correctly.

Updated to follow the same pattern used by the server-error and
client-error flashes in the generated layouts.

Closes #6624

Author: eagle-head

installer/templates/phx_web/components/core_components.ex.eex

@@ -37,7 +37,14 @@ defmodule <%= @web_namespace %>.CoreComponents do
   ## Examples
 
       <.flash kind={:info} flash={@flash} />
-      <.flash kind={:info} phx-mounted={show("#flash")}>Welcome Back!</.flash>
+      <.flash
+        id="welcome-back"
+        kind={:info}
+        phx-mounted={show("#welcome-back") |> JS.remove_attribute("hidden")}
+        hidden
+      >
+        Welcome Back!
+      </.flash>
   """
   attr :id, :string, doc: "the optional id of flash container"
   attr :flash, :map, default: %{}, doc: "the map of flash messages to display"