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
