Update io.md (#4470)

Add missing function brackets.

docs/io.md

@@ -8,7 +8,7 @@ The io() system
 ==================== 
 - If you need to ask the user a question, or print non-object content, use the io() system. 
 - A command callback gets access via `$this->io()`.
-- The main methods for gathering user input are `$this->io->choice()` and `$this->io()->confirm`.
+- The main methods for gathering user input are `$this->io()->choice()` and `$this->io()->confirm()`.
 - You may use any of the methods described in the [Symfony Style docs](https://symfony.com/doc/current/console/style.html).