DOC: fixed using IP.prompt_manager which is removed from IPython 5.x
- [x] closes #14003, #13639
- [ ] tests added / passed (there's no test on building docs)
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry (it's not user facing bug fix)

this PR was made during PyCon APAC, pandas sprint

thanks

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14004?src=pr) is 85.28% (diff: 100%)

> Merging [#14004](https://codecov.io/gh/pydata/pandas/pull/14004?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #14004   diff @@
==========================================
  Files           139        139          
  Lines         50241      50241          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42849      42849          
  Misses         7392       7392          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [6b7857b...cfd9499](https://codecov.io/gh/pydata/pandas/compare/6b7857b0154353f3fc1e80110876371cb10b2136...cfd94992134852d36877d5dc45f26c3960a838ba?src=pr)
 Thanks! 
Added a small comment. I also think a whatsnew notice is not needed, as it is not a user facing change/bug fix
 Thanks! Done what you said.
- revert to # comment
- no whatsnew notice
 @znmean Thanks!
