Updates attribute name in test assertion

Fixes an error in the test suite by updating the attribute name
used to access correspondence set length. It changes
`correspondence_set_` to `correspondence_set`.

Author: eclipse0922

python/test/test_symmetric_icp.py

@@ -166,7 +166,7 @@ def test_registration_symmetric_icp(self):
         result = o3d.pipelines.registration.registration_symmetric_icp(
             source, target, 0.1, np.eye(4), estimation, criteria)
 
-        assert len(result.correspondence_set_) > 0
+        assert len(result.correspondence_set) > 0
         assert result.fitness > 0.0
         assert result.inlier_rmse >= 0.0
 
@@ -197,7 +197,7 @@ def test_registration_symmetric_icp_with_robust_kernel(self):
         result = o3d.pipelines.registration.registration_symmetric_icp(
             source, target, 0.1, np.eye(4), estimation, criteria)
 
-        assert len(result.correspondence_set_) > 0
+        assert len(result.correspondence_set) > 0
         assert result.fitness > 0.0
 
     def test_registration_symmetric_icp_convergence(self):