DOC: Fix SageMaker example (#4598)

# Fix SageMaker example typing

Since #3249 a new type
`LLMContentHandler` is enforced for SageMaker Endpoints

Fixes #4168

docs/modules/models/llms/integrations/sagemaker.ipynb

@@ -93,7 +93,7 @@
     "from typing import Dict\n",
     "\n",
     "from langchain import PromptTemplate, SagemakerEndpoint\n",
-    "from langchain.llms.sagemaker_endpoint import ContentHandlerBase\n",
+    "from langchain.llms.sagemaker_endpoint import LLMContentHandler\n",
     "from langchain.chains.question_answering import load_qa_chain\n",
     "import json\n",
     "\n",
@@ -110,7 +110,7 @@
     "    template=prompt_template, input_variables=[\"context\", \"question\"]\n",
     ")\n",
     "\n",
-    "class ContentHandler(ContentHandlerBase):\n",
+    "class ContentHandler(LLMContentHandler):\n",
     "    content_type = \"application/json\"\n",
     "    accepts = \"application/json\"\n",
     "\n",