fix(service-worker): esm2015 points to wrong path (#20800)

The package.json esm2015 points to the wrong path.
"esm15" should be "esm2015"

Service Worker can't be compiled with use of Closure Compiler
PR Close #20800

Author: Doehl

packages/service-worker/package.json

@@ -4,7 +4,7 @@
   "description": "Angular - service worker tooling!",
   "main": "./bundles/service-worker.umd.js",
   "module": "./esm5/service-worker.js",
-  "es2015": "./esm15/service-worker.js",
+  "es2015": "./esm2015/service-worker.js",
   "typings": "./service-worker.d.ts",
   "author": "angular",
   "license": "MIT",