Fix grammar in plug guide (phoenixframework#3795)

guides/plug.md

@@ -290,7 +290,7 @@ defmodule HelloWeb.MessageController do
 end
 ```
 
-To make this all work, we converted the nested blocks of code and used `halt(conn)` whenever we reached a failure path. The `halt(conn)` functionality is essential here: it tells Plug that the next plug should not invoked.
+To make this all work, we converted the nested blocks of code and used `halt(conn)` whenever we reached a failure path. The `halt(conn)` functionality is essential here: it tells Plug that the next plug should not be invoked.
 
 At the end of the day, by replacing the nested blocks of code with a flattened series of plug transformations, we are able to achieve the same functionality in a much more composable, clear, and reusable way.