Fix average_checkpoints usage in docs
Fixes
## Checkpoints averaging
-The script `onmt-average-checkpoints` can be used to average the parameters of several checkpoints, usually increasing the model performance. For example:
+The `average_checkpoints` run type can be used to average the parameters of several checkpoints, usually increasing the model performance. For example:
```bash
onmt-main \
--config config/my_config.yml --auto_config \
- average-checkpoints \
+ average_checkpoints \
--output_dir run/baseline-enfr/avg \
--max_count 5
```
