address reviewer comment: update comment.

Change-Id: Ia5d8d2a9c7455099e9dd1023c311ce6702eb739d

Author: psunn

tensorflow_model_optimization/python/core/sparsity/keras/pruning_impl.py

@@ -241,7 +241,7 @@ def conditional_mask_update(self):
 
     def maybe_update_masks():
       if self._sparsity_m_by_n:
-        # only calculate m_by_n masks at step 1
+        # Update structured sparsity masks only at step 1
         return tf.math.equal(self._step_fn(), 1)
       else:
         return self._pruning_schedule(self._step_fn())[0]