Fix partial path traversal Java example Again

The original wouldn't compile, and the fix made by #11899 is sub-optimal.
This keeps the entire comparision using the Java `Path` object, which is optimal.

Signed-off-by: Jonathan Leitschuh <Jonathan.Leitschuh@gmail.com>

java/ql/test/query-tests/security/CWE-023/semmle/tests/PartialPathTraversalTest.java

@@ -225,6 +225,12 @@ public class PartialPathTraversalTest {
         }
     }
 
+    public void doesNotFlagOptimalSafeVersion(File dir, File parent) throws IOException {
+        if (!dir.toPath().normalize().startsWith(parent.toPath())) { // Safe
+            throw new IOException("Path traversal attempt: " + dir.getCanonicalPath());
+        }
+    }
+
     public void doesNotFlag() {
         "hello".startsWith("goodbye");
     }