DOC: Fixed warning about convert_objects in doc build
`convert_objects` doesn't exist anymore, and the example in the release notes when it was added is failing and generating a warning in the doc build:
```
Exception in /home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.11.0.rst at block ending on line None
Specify :okexcept: as an option in the ipython:: block to suppress this message
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-20-d11827bcdf31> in <module>
----> 1 s.convert_objects(convert_dates='coerce')
~/build/pandas-dev/pandas/pandas/core/generic.py in __getattr__(self, name)
   5095             if self._info_axis._can_hold_identifiers_and_holds_name(name):
   5096                 return self[name]
-> 5097             return object.__getattribute__(self, name)
   5098 
   5099     def __setattr__(self, name, value):
AttributeError: 'Series' object has no attribute 'convert_objects'
```

Open during the London sprint.

- [X] xref #24173
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Thanks for the PR! What warning were you getting? Any easy way to detect others? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26679?src=pr&el=h1) Report
> Merging [#26679](https://codecov.io/gh/pandas-dev/pandas/pull/26679?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1d7ad5fd7577f3da1c8eb19cf547f62d392405d0?src=pr&el=desc) will **increase** coverage by `48.56%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26679/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26679?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #26679       +/-   ##
===========================================
+ Coverage   41.84%    90.4%   +48.56%     
===========================================
  Files         174      174               
  Lines       50663    50663               
===========================================
+ Hits        21201    45803    +24602     
+ Misses      29462     4860    -24602
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.4% <ø> (?)` | |
| #single | `?` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26679?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/feather\_format.py](https://codecov.io/gh/pandas-dev/pandas/pull/26679/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2ZlYXRoZXJfZm9ybWF0LnB5) | `21.05% <0%> (-68.43%)` | :arrow_down: |
| [pandas/core/computation/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/26679/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vcHl0YWJsZXMucHk=) | `62.5% <0%> (-27.44%)` | :arrow_down: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/26679/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `63.82% <0%> (-25.51%)` | :arrow_down: |
| [pandas/io/clipboard/clipboards.py](https://codecov.io/gh/pandas-dev/pandas/pull/26679/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NsaXBib2FyZC9jbGlwYm9hcmRzLnB5) | `31.88% <0%> (-2.9%)` | :arrow_down: |
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/26679/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `17.61% <0%> (+1.68%)` | :arrow_up: |
| [pandas/util/\_test\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/26679/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3Rlc3RfZGVjb3JhdG9ycy5weQ==) | `93.22% <0%> (+5.08%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26679/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (+5.26%)` | :arrow_up: |
| [pandas/\_config/localization.py](https://codecov.io/gh/pandas-dev/pandas/pull/26679/diff?src=pr&el=tree#diff-cGFuZGFzL19jb25maWcvbG9jYWxpemF0aW9uLnB5) | `86% <0%> (+6%)` | :arrow_up: |
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/26679/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `96.85% <0%> (+10.23%)` | :arrow_up: |
| [pandas/compat/numpy/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/26679/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9udW1weS9fX2luaXRfXy5weQ==) | `93.1% <0%> (+13.79%)` | :arrow_up: |
| ... and [130 more](https://codecov.io/gh/pandas-dev/pandas/pull/26679/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26679?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/26679?src=pr&el=footer). Last update [1d7ad5f...27bb4de](https://codecov.io/gh/pandas-dev/pandas/pull/26679?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I think all the CI failures are unrelated, not sure what's broken there.

But looks like the warning is still in the docs. @nadebu can you see if you find what's wrong?

@WillAyd added more info to the description. @nadebu sorry, it have been working the last days on the documentation, and this has already been fixed in the meantime. Hopefully you can find another issue to work on! (there are some labeled with good-first-issue) Also, specifically for https://github.com/pandas-dev/pandas/issues/24173 (flake8 in the docs), there are still files left to do, so very welcome to open PRs for those. Sorry I missed all this conversation. My mail was being diverted and still familiarising myself with github's messaging.