Update comment in socket.ex (#5747)

Incorrect use of possessive as plural.

priv/templates/phx.gen.socket/socket.ex

@@ -42,7 +42,7 @@ defmodule <%= module %>Socket do
     {:ok, socket}
   end
 
-  # Socket id's are topics that allow you to identify all sockets for a given user:
+  # Socket IDs are topics that allow you to identify all sockets for a given user:
   #
   #     def id(socket), do: "user_socket:#{socket.assigns.user_id}"
   #