Fix argument typo in pad_packed_sequence docstring (pytorch#1300)

Author: BenFielding

torch/nn/utils/rnn.py

@@ -89,7 +89,7 @@ def pad_packed_sequence(sequence, batch_first=False):
  It is an inverse operation to :func:`pack_padded_sequence`.
 
  The returned Variable's data will be of size TxBx*, where T is the length
- of the longest sequence and B is the batch size. If ``batch_size`` is True,
+ of the longest sequence and B is the batch size. If ``batch_first`` is True,
  the data will be transposed into BxTx* format.
 
  Batch elements will be ordered decreasingly by their length.