Update main file

Author: sgrove

package.json

@@ -2,7 +2,7 @@
   "name": "netlify-plugin-netlify-graph",
   "version": "0.0.1",
   "description": "Netlify Graph build plugin",
-  "main": "src/index.mjs",
+  "main": "src/index.js",
   "files": [
     "src/**/*.js",
     "src/**/*.mjs",