Merge pull request #336 from Unity-Technologies/fix-3Dexception-typo

fix the exception typo, should be "3D" not "2D"

com.unity.perception/Runtime/GroundTruth/Labelers/BoundingBox3DLabeler.cs

         protected override void Setup()
         {
             if (idLabelConfig == null)
-                throw new InvalidOperationException("BoundingBox2DLabeler's idLabelConfig field must be assigned");
+                throw new InvalidOperationException("BoundingBox3DLabeler's idLabelConfig field must be assigned");
 
             m_AnnotationDefinition = DatasetCapture.RegisterAnnotationDefinition("bounding box 3D", idLabelConfig.GetAnnotationSpecification(),
                 "Bounding box for each labeled object visible to the sensor", id: new Guid(annotationId));