Fixed Broken Header

For some reason the Heading 1 cell cannot accept \infty latex.
I am sure this used to work; I have done some updates so perhaps
something changed?

13_HInfinity_Filters/HInfinity_Filters.ipynb

@@ -16,11 +16,10 @@
      ]
     },
     {
-     "cell_type": "heading",
-     "level": 1,
+     "cell_type": "markdown",
      "metadata": {},
      "source": [
-      "$H_\\infty$ filter"
+      " # $H_\\infty$ filter"
      ]
     },
     {
@@ -299,7 +298,6 @@
       "             [0., 1.]])\n",
       "\n",
       "f.H = array([[0., 1.]])\n",
-      "#f.x = array([[0., 0.]]).T\n",
       "f.G = array([[dt**2 / 2, dt]]).T\n",
       "\n",
       "f.P = 0.01\n",