change PURL to resolve to NCBI, not Ontobee

[cmungall]

NCBI is the authority, we should resolve there (just as PR resolves to the PRO website, GO to AmiGO, HPO to HPO/Monarch, etc).

E.g. https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=9606

[cmungall]

8 thumbs up let's do it

[anitacaron]

According to the README at the purl system, only OBO admins can change obo.yml, which is necessary when defining a custom term browser. I've created an issue there.