Update code_understanding.ipynb (#12309)

- **Description:** Super simple fix for colab link on
code_understanding.ipynb,
  - **Issue:** not applicable
  - **Dependencies:** none,
  - **Tag maintainer:** ,
  - **Twitter handle:** @kengoodridge

docs/docs/use_cases/question_answering/code_understanding.ipynb

@@ -14,7 +14,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "[![Open In Collab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/langchain-ai/langchain/blob/master/docs/docs/use_cases/code_understanding.ipynb)\n",
+    "[![Open In Collab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/langchain-ai/langchain/blob/master/docs/docs/use_cases/question_answering/code_understanding.ipynb)\n",
     "\n",
     "## Use case\n",
     "\n",