docs: update title of pypi notebook example to reflect use of the PyPI public dataset (#952)

In response to feedback on internal change 662899733.

Author: tswast

notebooks/dataframes/pypi.ipynb

@@ -25,7 +25,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# Analyzing Python dependencies with BigQuery DataFrames\n",
+    "# Analyzing package downloads from PyPI with BigQuery DataFrames\n",
     "\n",
     "In this notebook, you'll use the [PyPI public dataset](https://console.cloud.google.com/marketplace/product/gcp-public-data-pypi/pypi) and the [deps.dev public dataset](https://deps.dev/) to visualize Python package downloads for a package and its dependencies.\n",
     "\n",