Correct level of heading

Author: fangohr

book/15-visualising-data.ipynb

@@ -6932,7 +6932,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### The matplotlib.pyplot interface\n",
+    "## The matplotlib.pyplot interface\n",
     "\n",
     "This is the recommended way to use matplotlib for producing publication quality plots, or anything that needs some fine tuning: the object oriented approach of the `pyplot` interface makes it generally easier to tailor the plots that the state-driven `pylab` interface.\n",
     "\n",