fix title level (#663)

docs/source/notebooks/mmm/mmm_tvp_example.ipynb

@@ -4,7 +4,8 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## MMM with time-varying parameters (TVP)\n",
+    "(mmm_tv_intercept)=\n",
+    "# MMM with time-varying parameters (TVP)\n",
     "\n",
     "In classical marketing mix models, the effect of advertising (or other factors) on sales is assumed to be constant over time. Similarly, the baseline sales—what you would have sold without marketing—is also assumed to be constant. This is a simplification that _typically doesn't_ match reality. There will be times when, for various reasons, your ads are more effective, or when your product just _sells better_.\n",
     "\n",