Lumen blog link fix (#88)

* Updates for Lumen AI announcement post

* Fix link to Lumen Getting Started guide

Author: philippjfr

posts/lumen_ai_announcement/index.ipynb

@@ -232,7 +232,7 @@
     "lumen-ai serve https://datasets.holoviz.org/windturbines/v1/windturbines.parquet --show\n",
     "```\n",
     "\n",
-    "which will open the UI! Check out [Getting Started](https://lumen.holoviz.org/getting_started/using_lumen_ai.html) for more info!\n",
+    "which will open the UI! Check out [Getting Started](https://lumen.holoviz.org/lumen_ai/getting_started/index.html) for more info!\n",
     "\n",
     "We are really excited to hear your feedback, feature requests and more. So:\n",
     "\n",