Fix dataset paths to follow the tutorials' descriptions

I get errors trying to run the codes along with Deep Learning Tutorials.

e.g.)

    $ python code/logistic_sgd.py
    Downloading data from http://deeplearning.net/data/mnist/mnist.pkl.gz
    Traceback (most recent call last):
      File "code/logistic_sgd.py", line 372, in <module>
        sgd_optimization_mnist()
      File "code/logistic_sgd.py", line 234, in sgd_optimization_mnist
        datasets = load_data(dataset)
      File "code/logistic_sgd.py", line 164, in load_data
        urllib.urlretrieve(origin, dataset)
      File "/usr/local/Cellar/python/2.7.6/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib.py", line 94, in urlretrieve
        return _urlopener.retrieve(url, filename, reporthook, data)
      File "/usr/local/Cellar/python/2.7.6/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib.py", line 244, in retrieve
        tfp = open(filename, 'wb')
    IOError: [Errno 2] No such file or directory: '../data/mnist.pkl.gz'