fix: fix bug in associating tensorboard to an experiment

PiperOrigin-RevId: 501577576

Author: sararob

google/cloud/aiplatform/metadata/metadata.py

@@ -244,7 +244,7 @@ def set_experiment(
         current_backing_tb = experiment.backing_tensorboard_resource_name
 
         if not current_backing_tb and backing_tb:
-            experiment.assign_backing_tensorboard(tensorboard=backing_tensorboard)
+            experiment.assign_backing_tensorboard(tensorboard=backing_tb)
 
         self._experiment = experiment