Doc fixes: issue with [1..256] in the example

I was trying out this example from the cryptohash documentation:

```haskell
import qualified Crypto.Hash.SHA1 as SHA1

main = putStrLn $ show $ SHA1.hash (Data.ByteString.pack [1..256])
```

and I was puzzled because it seemed to be using a zero-length bytestring.  That turns out to be the case, because Data.ByteString.pack takes a [Word8], and:

```haskell
[1..256] :: [Word8]
```

is the same as:

```haskell
[1..0] :: [Word8]
```

which is the same as:

```haskell
[] :: [Word8]
```

I assume the intent was to hash 256 bytes, so I've submitted a patch to hash [0..255] instead of [1..256].