[pyupgrade] Make example error out-of-the-box (UP023) (#19291)

## Summary

Part of #18972

This PR makes [deprecated-c-element-tree
(UP023)](https://docs.astral.sh/ruff/rules/deprecated-c-element-tree/#deprecated-c-element-tree-up023)'s
example error out-of-the-box. I have no clue why the `import
xml.etree.cElementTree` and `from xml.etree import cElementTree` cases
are specifically carved out if they do not have an `as ...`, but the
tests explicitly call this out, and that's how it is in `pyupgrade`'s
source as well.


b5c5f710fc/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP023.py (L23-L31)

[Old example](https://play.ruff.rs/632b8ce1-393d-45e5-9504-5444ae71a0d8)
```py
from xml.etree import cElementTree
```

[New example](https://play.ruff.rs/fef4d378-8c54-41b2-8778-2d02bcbbd7d3)
```py
from xml.etree import cElementTree as ET
```

The "Use instead" section was also updated similarly.

## Test Plan

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

N/A, no functionality/tests affected

crates/ruff_linter/src/rules/pyupgrade/rules/deprecated_c_element_tree.rs

@@ -14,12 +14,12 @@ use crate::{AlwaysFixableViolation, Edit, Fix};
 ///
 /// ## Example
 /// ```python
-/// from xml.etree import cElementTree
+/// from xml.etree import cElementTree as ET
 /// ```
 ///
 /// Use instead:
 /// ```python
-/// from xml.etree import ElementTree
+/// from xml.etree import ElementTree as ET
 /// ```
 ///
 /// ## References