Fix API link to retrieve AFDB structures

[sillitoe]

The API to download structures from AFDB has changed:

from:
https://alphafold.ebi.ac.uk/files/AF-A0A000-F1-model_v4.cif

to:
https://alphafold.ebi.ac.uk/files/AF-A0A000-F1-model_v6.cif