chore(i18n): correct name of i18n extract command (angular#2511)

Author: wardbell

public/docs/_examples/package.json

@@ -21,7 +21,7 @@
     "build:webpack": "rimraf dist && webpack --config config/webpack.prod.js --bail",
     "build:cli": "ng build",
     "build:aot": "ngc -p tsconfig-aot.json && rollup -c rollup.js",
-    "extract": "ng-xi18n"
+    "i18n": "ng-xi18n"
   },
   "keywords": [],
   "author": "",