DOC: Modify astype copy=False example to work across platforms

[jschendel]

• passes git diff upstream/master -u -- "*.py" | flake8 --diff

Very minor, just changed 'int' -> 'int64'.

The example as previously written does not work on Windows, since astype('int') converts to int32 instead of int64. The Series constructor defaults to int64, so on Windows the astype ends up making a copy since the dtype changes, and thus the propagation to s1 doesn't occur.

This shouldn't impact Linux or Mac, since astype('int') converts to int64 on those platforms, so astype('int64') should be equivalent. I suppose this might not work for people using 32 bit distributions (?), but this should have wider coverage of potential readers of the docs, and is less confusing than using something like 'intp'.

[jreback]

thanks!