Broken example in coordinates docs

`distances.rst` contains the following example, that doesn't work for me:

```
    >>> from astropy.coordinates import CartesianPoints
    >>> ICRSCoordinates(x=568.7129, y=107.3009, z=507.8899, unit=u.kpc)
    <ICRSCoordinates RA=10.68458 deg, Dec=41.26917 deg, Distance=7.7e+02 kpc>
    >>> cp = CartesianPoints(x=568.7129, y=107.3009, z=507.8899, unit=u.kpc)
```

This raises the exception `__new__() got an unexpected keyword argument 'x'`.