[pylint] Make example error out-of-the-box (PLE1507) (#19288)

## Summary

Part of #18972

This PR makes [invalid-envvar-value
(PLE1507)](https://docs.astral.sh/ruff/rules/invalid-envvar-value/#invalid-envvar-value-ple1507)'s
example error out-of-the-box.

[Old example](https://play.ruff.rs/a46a9bca-edd5-4474-b20d-e6b6d87291ca)
```py
os.getenv(1)
```

[New example](https://play.ruff.rs/8348d32d-71fa-422c-b228-e2bc343765b1)
```py
import os

os.getenv(1)
```

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/pylint/rules/invalid_envvar_value.rs

@@ -18,11 +18,15 @@ use crate::checkers::ast::Checker;
 ///
 /// ## Example
 /// ```python
+/// import os
+///
 /// os.getenv(1)
 /// ```
 ///
 /// Use instead:
 /// ```python
+/// import os
+///
 /// os.getenv("1")
 /// ```
 #[derive(ViolationMetadata)]