Fix tutorial 306

Former-commit-id: 5f465bdc7ccb1c47aff45c95de57ac9a586115c4

python/tutorial/306_EigenDecomposition.py

@@ -60,6 +60,7 @@ def key_down(viewer, key, mod):
         viewer.data.compute_normals()
         viewer.data.set_colors(C)
         return True
+    return False
 
 
 viewer.callback_key_down = key_down