fix: Non-consecutive headings

Author: liamjberrisford

individual_modules/improve_your_r_code/microbenchmark.ipynb

@@ -172,7 +172,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### Exercise\n",
+    "## Exercise\n",
     "Try microbenchmarking one of your own functions."
    ]
   }