chore: missing links in rag sparse example (#455)

## Problem

Add missing links

## Type of Change

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update
- [ ] Infrastructure change (CI configs, etc)
- [ ] Non-code change (docs, etc)
- [ ] None of the above: (explain here)

Author: jamescalam

learn/generation/langchain/rag-sparse-query-expansion.ipynb

@@ -1,5 +1,13 @@
 {
   "cells": [
+    {
+      "cell_type": "markdown",
+      "id": "a4da881e",
+      "metadata": {},
+      "source": [
+        "[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/pinecone-io/examples/blob/master/learn/generation/langchain/rag-sparse-query-expansion.ipynb) [![Open nbviewer](https://raw.githubusercontent.com/pinecone-io/examples/master/assets/nbviewer-shield.svg)](https://nbviewer.org/github/pinecone-io/examples/blob/master/learn/generation/langchain/rag-sparse-query-expansion.ipynb)"
+      ]
+    },
     {
       "cell_type": "markdown",
       "id": "9abb5eee-b493-42f2-84c4-2615286cfeb5",
@@ -29,6 +37,16 @@
         "Let's see how we can implement this approach with Pinecone's sparse vector database."
       ]
     },
+    {
+      "cell_type": "markdown",
+      "id": "92b2fffc",
+      "metadata": {},
+      "source": [
+        "### Before you begin\n",
+        "\n",
+        "You'll need to get an [OpenAI API key](https://platform.openai.com/account/api-keys) and [Pinecone API key](https://app.pinecone.io)."
+      ]
+    },
     {
       "cell_type": "markdown",
       "id": "R6GUuNTJZK99",