update the PDB url

Hi @martin-steinegger and all the members,
I found a outdated URL in the following code:
```
# Fetch PDBs
if [ ! -d tmp/pdb ]; then
    curl https://wwwuser.gwdg.de/~compbiol/foldseek/scp40pdb.tar.gz | tar -xz -C tmp
fi
```

If should be updated as:
```
https://wwwuser.gwdg.de/~compbiol/foldseek/scop40pdb.tar.gz
```