Fix mnist loader

[c-bata]

I download mnist data by examples/mnist/download.sh, then execute convnet example. But I got following errors:

$ ./bin/gorgonia_convnet
2020/04/19 22:10:35 Unable to read Labels: open /Users/a14737/go/src/github.com/c-bata/goptuna/_examples/gorgonia_convnet/data/train-labels.idx1-ubyte: no such file or directory

The reason why I got this error is just missed file name. download.sh generated a file train-labels-idx1-ubyte, not train-labels.idx1-ubyte. I fix this problem on this pull request.

[coveralls]

Coverage remained the same at 63.973% when pulling ad68940 on c-bata:fix-mnist-loader into cb4f110 on gorgonia:master.

[owulveryck]

Good catch, thank you.
We need to fix the tutorial on the website accordingly.
I will create an issue as a reminder