Language Reference: Section on objects mixes up dodo bird example and pigeon example

This is probably a simple editing error due to some changes that took place as the documentation was being written.

[Defining Objects](https://pkl-lang.org/main/current/language-reference/index.html#defining-objects) gives an example with dodo birds:

```
dodo {
name = "Dodo"
taxonomy {
`class` = "Aves"
}
}
```

But then a subsequent paragraph mentions a different example (emphasis added by me):

> Like all values, objects are immutable, which is just a fancy (and short!) way to say that their properties never change. **So what happens when Pigeon moves to a different street?** Do we have to construct a new object from scratch?

I think the emphasized sentence needs to be rewritten to refer to the dodo bird vs the tortoise in the following section.

Open the Language Reference at the Defining Objects section using the linked documentation URL and compare the dodo bird example with the following tortoise section. Update the paragraph that asks what happens when Pigeon moves so its example references the dodo bird and the tortoise consistently. Done means the prose matches the surrounding examples without changing the explanation.

Written by the indexing model from the issue text.