update the to_string

Author: SYaoJun

geaflow-ai/src/main/java/org/apache/geaflow/ai/index/vector/TraversalVector.java

@@ -113,6 +113,9 @@ public String toString() {
             if (i > 0) {
                 sb.append(i % 3 == 0 ? "; " : "-");
             }
+            if (i % 3 == 2) {
+                sb.append(">");
+            }
             sb.append(vec[i]);
         }
         return sb.append('}').toString();

geaflow-ai/src/test/java/org/apache/geaflow/ai/GraphMemoryTest.java

@@ -230,7 +230,7 @@ public void testTraversalVectorToString() {
         );
         String str = vector.toString();
 
-        assertEquals(str, "TraversalVector{vec=Alice-knows-Bob}");
+        assertEquals(str, "TraversalVector{vec=Alice-knows->Bob}");
     }
 
     @Test