Update sequence.md.
* Update sequence.md.
Fix value added for padding and add explanation on truncation.
* fix typo
## pad_sequences
```python
-keras.preprocessing.sequence.pad_sequences(sequences, maxlen=None, dtype='int32')
+keras.preprocessing.sequence.pad_sequences(sequences, maxlen=None, dtype='int32',
+ padding='pre', truncating='pre', value=0.)
```
-Transform a list of `nb_samples sequences` (lists of scalars) into a 2D Numpy array of shape `(nb_samples, nb_timesteps)`. `nb_timesteps` is either the `maxlen` argument if provided, or the length of the longest sequence otherwise. Sequences that are shorter than `nb_timesteps` are padded with zeros at the end.
+Transform a list of `nb_samples` sequences (lists of scalars) into a 2D Numpy array of shape `(nb_samples, nb_timesteps)`. `nb_timesteps` is either the `maxlen` argument if provided, or the length of the longest sequence otherwise. Sequences that are shorter than `nb_timesteps` are padded with `value` at the end. Sequences longer than `nb_timesteps` are truncated so that it fits the desired length. Position where padding or truncation happens is determined by `padding` or `truncating`, respectively.
- __Return__: 2D Numpy array of shape `(nb_samples, nb_timesteps)`.
