Quickstart prefix filter example is wrong

Example given:

```
curl "https://query.eventdata.crossref.org/events?rows=10000&filter=source:reddit,prefix:10.1371"
```
Should probably be:

```
curl "https://query.eventdata.crossref.org/events?rows=10000&filter=source:reddit,obj-id.prefix:10.1371"
```