Update metadata.json: rename subsets to match official HF config names

metadata.json

@@ -2,7 +2,7 @@
   "name": "MathVerse",
   "release_date": "2024-03-01",
   "subsets": {
-    "main": {
+    "testmini": {
       "language": [
         "en"
       ],
@@ -53,7 +53,7 @@
         }
       }
     },
-    "text_only": {
+    "testmini_text_only": {
       "language": [
         "en"
       ],