Update saving-your-data.rst

The set() section shouldn't speak on behalf of save(). 

The set() section incorrectly implied that the example would save *only* the changed fields. In fact, the example saves the entire record.

Better to let the save() section speak for itself.
Merge pull request #719 from mmapes/master

Update saving-your-data.rst
[fr] Follow #719

Update saving-your-data.rst