DOC: correct rpy2 examples (GH15142)
To fix the current doc build errors (https://github.com/pandas-dev/pandas/pull/15142#issuecomment-274346255)
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15450?src=pr&el=h1) Report
> Merging [#15450](https://codecov.io/gh/pandas-dev/pandas/pull/15450?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f65a6415f15d438432cc6954ead61b052c5d4d60?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15450      +/-   ##
==========================================
+ Coverage   90.37%   90.37%   +<.01%     
==========================================
  Files         135      135              
  Lines       49463    49463              
==========================================
+ Hits        44702    44703       +1     
+ Misses       4761     4760       -1
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15450?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/f65a6415f15d438432cc6954ead61b052c5d4d60...7594376f0a246712e9979e8472c52587b056bac2?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.36% <ø> (+0.33%)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15450?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15450?src=pr&el=footer). Last update [f65a641...7594376](https://codecov.io/gh/pandas-dev/pandas/compare/f65a6415f15d438432cc6954ead61b052c5d4d60...7594376f0a246712e9979e8472c52587b056bac2?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). Doc build now passes on this.