Fix a `np.random.np.random` typo in "computation.rst" in document.
There's `np.random.np.random` in /doc/source/user_guide/computation.rst, which I believe is a typo. But the weird thing is there's actually `np.random.np` in numpy (1.16.4), but not in numpy (1.17.2). That's maybe why the doc build passed before. While I'm trying to build the doc locally with numpy 1.17.2, it failed.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
