Replace @ with / in packages_oneapi.yaml for hercules / orion / ursa

**Describe the bug**
`configs/sites/tier1/{hercules, orion, ursa}/packages_oneapi.yaml`

`intel-oneapi-mpi@2021.13.1` is not valid syntax for loading a module

`intel-oneapi-mpi@2021.13.1` --> `intel-oneapi-mpi/2021.13.1`

**System:**

- **hercules**
- **orion**
- **ursa**

**Additional context**
This is manually fixed, does not require a rebuild.
