Use CURIEs in spec docs

The spec mentions CURIEs, but includes examples like this:

https://github.com/ga4gh-discovery/data-connect/blob/develop/SPEC.md#search-example

```json
Request body:
{
"query": "SELECT * from search_postgres_pgpc.ontology.axiom WHERE to_term=?"
"parameters": [ "UBERON_0000464" ]
}
```

I would recommend using the UBERON CURIE, i.e. `UBERON:0000464` (and having a separate prefixmap that points to the OBO PURL)

Thanks,
Chris (Uberon developer)