Fix inconsistent headings for semantic router user guide (#336)

This fixes inconsistent headings

Author: paololazzari

docs/user_guide/08_semantic_router.ipynb

@@ -523,7 +523,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# Add route references"
+    "## Add route references"
    ]
   },
   {
@@ -558,7 +558,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# Get route references"
+    "## Get route references"
    ]
   },
   {
@@ -631,7 +631,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# Delete route references"
+    "## Delete route references"
    ]
   },
   {