DOC: remove references to deprecated numpy negation method
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13310?src=pr) is **84.22%**

> Merging [#13310](https://codecov.io/gh/pydata/pandas/pull/13310?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13310   diff @@
==========================================
  Files           138        138          
  Lines         50666      50666          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42672      42672          
  Misses         7994       7994          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [ae2ca83...3b46a1b](https://codecov.io/gh/pydata/pandas/compare/ae2ca83d31ebc2f95bb4f71b504f454ec399ee41...3b46a1b847013b0429bc6dcd2c58ea175c23b5cf)
 thanks!
