[`flake8-simplify`] Make example error out-of-the-box (`SIM401`) (#19110)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Part of #18972

This PR makes [enumerate-for-loop [if-else-block-instead-of-dict-get
(SIM401)](https://docs.astral.sh/ruff/rules/if-else-block-instead-of-dict-get/#if-else-block-instead-of-dict-get-sim401)'s
example error out-of-the-box

[Old example](https://play.ruff.rs/635629eb-7146-45a8-9e0c-4a0aa9446ded)
```py
if "bar" in foo:
    value = foo["bar"]
else:
    value = 0
```

[New example](https://play.ruff.rs/a1227ec9-05c2-4a22-800d-c76cb7abe249)
```py
foo = {}
if "bar" in foo:
    value = foo["bar"]
else:
    value = 0
```

The "Use instead" section was also updated similarly.

The docs for `SIM401` also has another section on the preview ternary
version, but it does not seem to check that the variable is a dict
(bug?) https://play.ruff.rs/c0feada8-a7fe-43f7-b57e-c10520fdcdca

## Test Plan

<!-- How was it tested? -->

N/A, no functionality/tests affected

crates/ruff_linter/src/rules/flake8_simplify/rules/if_else_block_instead_of_dict_get.rs

@@ -27,6 +27,7 @@ use crate::{Edit, Fix, FixAvailability, Violation};
 ///
 /// ## Example
 /// ```python
+/// foo = {}
 /// if "bar" in foo:
 ///     value = foo["bar"]
 /// else:
@@ -35,6 +36,7 @@ use crate::{Edit, Fix, FixAvailability, Violation};
 ///
 /// Use instead:
 /// ```python
+/// foo = {}
 /// value = foo.get("bar", 0)
 /// ```
 ///