message: Small doc example fix in add_metric.
diff: @@ -1364,8 +1364,7 @@ class Layer(module.Module, version_utils.LayerVersionSelector):
  self.mean = metrics_module.Mean(name='metric_1')
  def call(self, inputs):
  - # Provide same name as in the instance created in __init__
  - self.add_metric(self.mean(x), name='metric_1')
  + self.add_metric(self.mean(x))
  self.add_metric(math_ops.reduce_sum(x), name='metric_2')
  return inputs
  ```
  