message: Update math_ops.py
  Corrected documentation of tf.reduce_mean()
diff: @@ -1437,7 +1437,7 @@ def reduce_mean(input_tensor,
  input_tensor: The tensor to reduce. Should have numeric type.
  axis: The dimensions to reduce. If `None` (the default),
  reduces all dimensions. Must be in the range
  - `[-rank(input_tensor), rank(input_tensor))`.
  + `[-rank(input_tensor), rank(input_tensor)]`.
  keepdims: If true, retains reduced dimensions with length 1.
  name: A name for the operation (optional).
  reduction_indices: The old (deprecated) name for axis.
  