fix: test variable renaming (#1007)

### Description

Trivial renaming in one test case to match the thing being tested.

### Checklist

Delete any checklist items that do not apply (e.g. if your change is
minor, it may not require documentation updates).

- [x] Pull request title follows [conventional
commits](https://www.conventionalcommits.org/en/v1.0.0/)
- [x] Pre-commit hooks pass (`prek run --all-files`)

Author: bradh

crates/extensions/src/authentication.rs

@@ -180,8 +180,8 @@ mod tests {
 
     #[test]
     fn item() {
-        let collection: Item = stac::read("data/auth/item.json").unwrap();
-        let authentication: Authentication = collection.extension().unwrap();
+        let item: Item = stac::read("data/auth/item.json").unwrap();
+        let authentication: Authentication = item.extension().unwrap();
         let _ = authentication.schemes.get("none").unwrap();
     }