message: Correct docstring in error_interpolation.
  The docstring previously referenced the old format for tags.
diff: @@ -267,8 +267,8 @@ def compute_field_dict(op):
  def interpolate(error_message, graph):
  """Interpolates an error message.
  - The error message can contain tags of the form ^^type:name^^ which will
  - be replaced.
  + The error message can contain tags of the form `{{type name}}` which will be
  + replaced.
  Args:
  error_message: A string to interpolate.
  