set correct field name

Co-Authored-By: Mu-An 慕安 <me@muanchiou.com>

Author: koddsson

package.json

@@ -5,7 +5,7 @@
   "repository": "github/details-dialog-element",
   "main": "dist/index.umd.js",
   "module": "dist/index.esm.js",
-  "files": "index.d.ts",
+  "types": "index.d.ts",
   "scripts": {
     "clean": "rm -rf dist",
     "lint": "github-lint",