Use into_owned over to_string for Cow<str> (#39024)

This removes unnecessary allocations when the `Cow` is already owned


Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/eval/src/examples/mod.rs

@@ -106,7 +106,7 @@ impl DeclarativeExample {
     }
 
     pub fn name_from_path(path: &Path) -> String {
-        path.file_stem().unwrap().to_string_lossy().to_string()
+        path.file_stem().unwrap().to_string_lossy().into_owned()
     }
 }