KeyError when Loading TREC CAsT datasets

**Describe the bug**
Hi! I was trying to access version 3 of the TREC CAsT corpus but got a `KeyError: 'trec-cast/v3'` when I ran:

```python
import ir_datasets
dataset = ir_datasets.load("trec-cast/v3")
for doc in dataset.docs_iter():
print(doc)
break
```

Of the datasets listed on the listed on the [TREC CAsT Page](https://ir-datasets.com/trec-cast.html#trec-cast), it was only possible to load

```
trec-cast', 'trec-cast/v0', 'trec-cast/v0/train', 'trec-cast/v0/train/judged', 'trec-cast/v1', 'trec-cast/v1/2019', 'trec-cast/v1/2019/judged', 'trec-cast/v1/2020', 'trec-cast/v1/2020/judged'
```

**Affected dataset(s)**

TREC CAsT

**To Reproduce**
Run the following:

```python
import ir_datasets
dataset = ir_datasets.load("trec-cast/v3")
for doc in dataset.docs_iter():
print(doc)
break
```

This should result in:

```bash
File "...ir_datasets/util/registry.py", line 26, in __getitem__
result = self._registered[key]
~~~~~~~~~~~~~~~~^^^^^
KeyError: 'trec-cast/v3'
```

**Expected behavior**
The first document in the dataset printed out to the terminal