[MRG] DOC Refactor example to remove ambiguity in understanding param order

Fixes #3267
That implementation of `__iter__` doesn't seem to sort the output... 

From the below you could see that the order is still arbitrary... but consistent for a given dict configuration...

```
>>> for param_dict in ParameterGrid(dict(b=range(2), c=range(2))):
...     print param_dict
... 
{'c': 0, 'b': 0}
{'c': 1, 'b': 0}
{'c': 0, 'b': 1}
{'c': 1, 'b': 1}
```
Maybe we should use [`OrderedDict`](https://docs.python.org/3/library/collections.html#collections.OrderedDict) there?