Fix typo in releases.json (#739)

fix typo

Author: AyodeAwe

_data/releases.json

@@ -48,7 +48,7 @@
       "days": "2"
     }
   },
-  "next-nightly": {
+  "next_nightly": {
     "version": "26.04",
     "ucxx_version": "0.49",
     "cudf_dev": {