docs: Astra DB vectorstore, adjust syntax for automatic-embedding example (#22833)

Description: Adjusting the syntax for creating the vectorstore
collection (in the case of automatic embedding computation) for the most
idiomatic way to submit the stored secret name.

Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>

docs/docs/integrations/vectorstores/astradb.ipynb

@@ -245,7 +245,7 @@
     "    provider=\"openai\",\n",
     "    model_name=\"text-embedding-3-small\",\n",
     "    authentication={\n",
-    "        \"providerKey\": \"MY_OPENAI_API_KEY.providerKey\",\n",
+    "        \"providerKey\": \"MY_OPENAI_API_KEY\",\n",
     "    },\n",
     ")\n",
     "\n",