Fix the env variable for OpenAI Base Url (#639)

For using Azure OpenAI API, we need to set multiple env vars. But as can
be seen in openai package
[here](48b69293a3/openai/__init__.py (L35)),
the env var for setting base url is named `OPENAI_API_BASE` and not
`OPENAI_API_BASE_URL`. This PR fixes that part in the documentation.

docs/modules/llms/integrations/azure_openai_example.ipynb

@@ -20,7 +20,7 @@
     "# The API version you want to use: set this to `2022-12-01` for the released version.\n",
     "export OPENAI_API_VERSION=2022-12-01\n",
     "# The base URL for your Azure OpenAI resource.  You can find this in the Azure portal under your Azure OpenAI resource.\n",
-    "export OPENAI_API_BASE_URL=https://your-resource-name.openai.azure.com\n",
+    "export OPENAI_API_BASE=https://your-resource-name.openai.azure.com\n",
     "# The API key for your Azure OpenAI resource.  You can find this in the Azure portal under your Azure OpenAI resource.\n",
     "export OPENAI_API_KEY=<your Azure OpenAI API key>\n",
     "```\n",