docs -- double link to asdf example

[teald]

The documentation repeats the same link twice in the first section of Basic Structure of a GWCS object. The two bullet points both go to Save a WCS object as a pure ASDF file.

The preceding text indicates they should be different, but I'm not sure what it should link to. Here's a link to the relevant part of index.rst. Minor issue, hopefully an easy fix 😄

[nden]

Documentation needs to be cleaned up. New versions of asdf do not allow saving to FITS files.