Fix docstring for lz4.compress_block_bound

[ods]

Right now it's copy-pasted from compress_block_into method:

>>> print(cramjam.lz4.compress_block_bound.__doc__)

Determine the size of a buffer which is guaranteed to hold the result of block compression, will error if
data is too long to be compressed by lz4.

Python Example
--------------
```python
>>> cramjam.lz4.compress_block_into(
...     b'some bytes here',
...     output=output_buffer,
...     mode=Option[str],
...     acceleration=Option[int],
...     compression=Option[int],
...     store_size=Option[bool]
... )
```

[milesgranger]

super, thank you!