gh-116535: Fix distracting "TypeError" in example code

[dec1]

Fix: TypeError: 'int' object is not iterable when you call the sample code:

Note this is not the advertised (and expected) ValueError, which is fixed by using field

@dataclass
class D:
    x: list = []      # This code no longer raises ValueError
    def add(self, element):
        self.x += element # calling this (with an int element) raises TypeError

• Issue: Mutable default values example - TypeError: 'int' object is not iterable #116535

---

📚 Documentation preview 📚: https://cpython-previews--116538.org.readthedocs.build/

[ghost]

All commit authors signed the Contributor License Agreement.

[Privat33r-dev]

LGTM, thanks for the contribution

[miss-islington-app]

Thanks @dec1 for the PR, and @ericvsmith for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[bedevere-app]

GH-116551 is a backport of this pull request to the 3.12 branch.

[bedevere-app]

GH-116552 is a backport of this pull request to the 3.11 branch.