Update example code in contributor guide

This updates the UI example case code. The previous code snippet did not
compile successfully anymore.

src/doc/contrib/src/tests/writing.md

@@ -207,14 +207,15 @@ mod <case>;
 
 `tests/testsuite/<command>/<case>/mod.rs`:
 ```rust,ignore
-use cargo_test_support::prelude::*;
 use cargo_test_support::compare::assert_ui;
+use cargo_test_support::current_dir;
+use cargo_test_support::file;
+use cargo_test_support::prelude::*;
 use cargo_test_support::Project;
-use cargo_test_support::curr_dir;
 
 #[cargo_test]
 fn case() {
-    let project = Project::from_template(curr_dir!().join("in"));
+    let project = Project::from_template(current_dir!().join("in"));
     let project_root = project.root();
     let cwd = &project_root;
 
@@ -224,10 +225,10 @@ fn case() {
         .current_dir(cwd)
         .assert()
         .success()
-        .stdout_matches_path(curr_dir!().join("stdout.log"))
-        .stderr_matches_path(curr_dir!().join("stderr.log"));
+        .stdout_matches(file!("stdout.log"))
+        .stderr_matches(file!("stderr.log"));
 
-    assert_ui().subset_matches(curr_dir!().join("out"), &project_root);
+    assert_ui().subset_matches(current_dir!().join("out"), &project_root);
 }
 ```