fix CharDataset::__len__ off by one error

[fpgaminer]

I made an off by one mistake in my comments on issue #22, which unfortunately got rolled into 339f4e7. Sorry about that!

Verified by example:

>>> data = list(range(8))
>>> len(data)
8
>>> block_size = 4
>>> 
>>> 
>>> for i in range(len(data) - block_size):
...     print(i)
...     print(data[i:i+block_size+1])
... 
0
[0, 1, 2, 3, 4]
1
[1, 2, 3, 4, 5]
2
[2, 3, 4, 5, 6]
3
[3, 4, 5, 6, 7]
>>> for i in range(len(data) - (block_size+1)):
...     print(i)
...     print(data[i:i+block_size+1])
... 
0
[0, 1, 2, 3, 4]
1
[1, 2, 3, 4, 5]
2
[2, 3, 4, 5, 6]

[karpathy]

easy way to see this is that when block_size + 1 == len(data) the call to len should return 1. I actually spotted the issue in your PR and then copied it anyway into the code forgot about it, I think it was just too late.