refactor(package.json): Fix external license reference (#12)

Update package license for easier dist, no need for foreign reference with the license change to Apache-2.0

Author: tracker1

package.json

@@ -3,7 +3,7 @@
   "version": "4.0.0",
   "description": "Firebase JavaScript library for web and Node.js",
   "author": "Firebase <[email protected]> (https://firebase.google.com/)",
-  "license": "SEE LICENSE IN https://firebase.google.com/terms/",
+  "license": "Apache-2.0",
   "homepage": "https://firebase.google.com/",
   "keywords": [
     "authentication",