Fix license as SPDX code

package.json

@@ -11,7 +11,7 @@
     "type": "git",
     "url": "https://github.com/gaearon/prism.git"
   },
-  "license": "Apache 2",
+  "license": "Apache-2.0",
   "bugs": {
     "url": "https://github.com/gaearon/prism/issues"
   },