using consistent subdomain key

README.md

@@ -49,7 +49,7 @@ This makes the dataset particularly suitable for:
 | ----------------- | ------ | ----------------------------------------- |
 | `conversation_id` | string | Unique identifier for each QA pair.       |
 | `domain`          | string | Physics, Math, Chemistry, Biology.        |
-| `sub-domain`      | string | Specific discipline.                      |
+| `sub_domain`      | string | Specific discipline.                      |
 | `question`        | string | STEM problem statement (LaTeX supported). |
 | `answer`          | string | Deterministic ground-truth solution.      |
 
@@ -61,7 +61,7 @@ This makes the dataset particularly suitable for:
 {
   "conversation_id": "217998",
   "domain": "Physics",
-  "sub-domain": "Astrophysics",
+  "sub_domain": "Astrophysics",
   "question": "Consider a Navarro–Frenk–White (NFW) dark matter halo profile where...",
   "answer": "\( \frac{4GM_{0}}{r_{0}} + \frac{16\pi Gk}{r_{0}}\left[ \ln\left(\frac{r_{0}}{r_{s}}\right) + 0.31 \right] \)"
 }