[`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.
https://github.com/astral-sh/ruff/blob/b5c5f710fc12b5c512a2e5351684b8ffdf33761f/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
