Fixed error in latex

It rendered fine in the notebook, but \text{foo_bar} was causing
an error in the pdf generation. Switching to \mathtt{foo\_bar}
mostly fixes it (though in the notebook it renders as -, not _.
Oye.

10-Unscented-Kalman-Filter.ipynb

@@ -1708,7 +1708,7 @@
    "source": [
     "To include the doppler we need to add the velocity in $x$ and $y$ into the measurement. The `ACSim` class stores the velocity in the data member `vel`. To perform the Kalman filter update we just need to call `update` with a list containing the bearing, distance, and velocity in $x$ and $y$:\n",
     "\n",
-    "$$z = [\\text{slant_range}, \\text{bearing}, \\dot x, \\dot y]$$\n",
+    "$$z = [\\mathtt{slant\\_range}, \\text{bearing}, \\dot x, \\dot y]$$\n",
     "\n",
     "The measurement contains four values so the measurement function also needs to return four values. The slant range and bearing will be computed as before, but we do not need to compute the velocity in $x$ and $y$ as they are provided in the state estimate."
    ]