[Console] Added a cookbook entry on invoking other commands

This is a follow-up to an issue I've opened in [symfony/symfony](https://github.com/symfony/symfony/issues/12495). It describes how to find and execute other commands from a Symfony `Command`.
I don't know if the `:` is needed here.

Instead of this:

```
these commands using the :class:`Symfony\\Component\\Console\\Application <Symfony\\Component\\Console\\Application>`:'s ``find`` method.
```

Use this?

```
these commands using the :class:`Symfony\\Component\\Console\\Application <Symfony\\Component\\Console\\Application>`'s ``find`` method.
```