DOC: fix docstring for DataFrame.interpolate

In the old docstring, `'index'` is listed as a valid value for the `method` kwarg for `DataFrame.interpolate`. It looks like a typo, because there was no comma after `'index'`, and the explanation for `'index'` corresponded to `'values'`. Furthermore, it appears `'index'` is not a valid value for `method`:

```python
In [67]: df = pd.DataFrame([[1], [2], [np.nan], [3]], index=[0, 0.7, 1.1, 4])

In [68]: df
Out[68]: 
      0
0.0   1
0.7   2
1.1 NaN
4.0   3

In [69]: df.interpolate(method="index")
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-69-4cef75cee92e> in <module>()
----> 1 df.interpolate(method="index")

/home/david/miniconda3/envs/frackoptima/lib/python3.4/site-packages/pandas/core/generic.py in interp
olate(self, method, axis, limit, inplace, downcast, **kwargs)                                      
   2580                                           inplace=inplace,
   2581                                           downcast=downcast,
-> 2582                                           **kwargs)
   2583         if inplace:
   2584             if axis == 1:

/home/david/miniconda3/envs/frackoptima/lib/python3.4/site-packages/pandas/core/internals.py in inte
rpolate(self, **kwargs)                                                                            
   2195 
   2196     def interpolate(self, **kwargs):
-> 2197         return self.apply('interpolate', **kwargs)
   2198 
   2199     def shift(self, **kwargs):

/home/david/miniconda3/envs/frackoptima/lib/python3.4/site-packages/pandas/core/internals.py in appl
y(self, f, axes, filter, do_integrity_check, **kwargs)                                             
   2162                                                  copy=align_copy)
   2163 
-> 2164             applied = getattr(b, f)(**kwargs)
   2165 
   2166             if isinstance(applied, list):

/home/david/miniconda3/envs/frackoptima/lib/python3.4/site-packages/pandas/core/internals.py in inte
rpolate(self, method, axis, index, values, inplace, limit, fill_value, coerce, downcast, **kwargs) 
    667                                      **kwargs)
    668 
--> 669         raise ValueError("invalid method '{0}' to interpolate.".format(method))
    670 
    671     def _interpolate_with_fill(self, method='pad', axis=0, inplace=False,

ValueError: invalid method 'index' to interpolate.

In [70]: df.interpolate(method="values")
Out[70]: 
            0
0.0  1.000000
0.7  2.000000
1.1  2.121212
4.0  3.000000
```