chore: refer directly to index.js file

Doesn't matter for Node, but tools like `unpkg` and `runpkg` gets
confused.

Author: SimenB

package.json

@@ -14,7 +14,7 @@
     "email": "hello@jkimbo.com",
     "url": "jkimbo.com"
   },
-  "main": "lib/",
+  "main": "lib/index.js",
   "files": [
     "docs/",
     "lib/"