docs: git dependency name correction (#20662)

This PR corrects the name of the `git` python package to `GitPython`.

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

docs/docs/use_cases/code_understanding.ipynb

@@ -45,7 +45,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "%pip install --upgrade --quiet langchain-openai tiktoken langchain-chroma langchain git\n",
+    "%pip install --upgrade --quiet langchain-openai tiktoken langchain-chroma langchain GitPython\n",
     "\n",
     "# Set env var OPENAI_API_KEY or load from a .env file\n",
     "# import dotenv\n",