Fix 'equal_to' knowledge base relationship

The mapping between knowledge base relationships and LangPro prover input is not correct in `backend/problem/utils.py`. Currently, `equal` and `subset` are both converted into single `isa_wn(...)` items and for `subset`, this is correct. For `equal`, however, there should be two.

```
Input:
{
"id": "1",
"entity1": "Hesperus",
"entity2": "Phosphorus",
"relationship": "equal"
}

Current output: "isa_wn(Hesperus, Phosphorus)"
Expected output: ["isa_wn(Hesperus, Phosporus)", "isa_wn(Phosphorus, Hesperus)"]
```