message: optimizer_v2: Update error message
  Remove suggestion about private method.
diff: @@ -441,8 +441,8 @@ class OptimizerV2(trackable.Trackable):
  if distribute_ctx.in_cross_replica_context():
  raise RuntimeError(
  "`apply_gradients() cannot be called in cross-replica context. "
  - "Use `_distributed_apply()` instead or enter replica context "
  - "by using `tf.distribute.Strategy.experimental_run_v2`.")
  + "Use `tf.distribute.Strategy.experimental_run_v2` to enter replica "
  + "context.")
  apply_state = self._prepare(var_list)
  return distribute_ctx.get_replica_context().merge_call(
  