Update URL Link (#251)

Seems that there is an errata on the URL blog address

Author: CarlesCG

supporting-blog-content/introducing-retrievers/retrievers_intro_notebook.ipynb

@@ -27,7 +27,7 @@
   {
    "cell_type": "markdown",
    "source": [
-    "This notebook allows you to run the exampls from the [Search Labs blog - Introducing Retrievers -  Search All the Things!](https://www.elastic.co/search-labs/blog/introducing-retrievers)\n",
+    "This notebook allows you to run the exampls from the [Search Labs blog - Introducing Retrievers -  Search All the Things!](https://www.elastic.co/search-labs/blog/elasticsearch-retrievers)\n",
     "\n",
     "In this notebook you will:\n",
     "- Download IMDB dataset from Kaggle\n",
@@ -1047,4 +1047,4 @@
    ]
   }
  ]
-}
+}