Uses \times instead of x for latex maths

The box defined should be $80 \times 100$ not $80x100$ - I think!

overview-ns-forw.ipynb

@@ -82,7 +82,7 @@
   {
    "cell_type": "markdown",
    "source": [
-    "The inflow will be used to inject smoke into a second centered grid `smoke` that represents the marker field $d$ from above. Note that we've defined a `Box` of size $100x80$ above. This is the physical scale in terms of spatial units in our simulation, i.e., a velocity of magnitude $1$ will move the smoke density by 1 unit per 1 time unit, which may be larger or smaller than a cell in the discretized grid, depending on the settings for `x,y`. You could parametrize your simulation grid to directly resemble real-world units, or keep appropriate conversion factors in mind. \n",
+    "The inflow will be used to inject smoke into a second centered grid `smoke` that represents the marker field $d$ from above. Note that we've defined a `Box` of size $100\times80$ above. This is the physical scale in terms of spatial units in our simulation, i.e., a velocity of magnitude $1$ will move the smoke density by 1 unit per 1 time unit, which may be larger or smaller than a cell in the discretized grid, depending on the settings for `x,y`. You could parametrize your simulation grid to directly resemble real-world units, or keep appropriate conversion factors in mind. \n",
     "\n",
     "The inflow sphere above is already using the \"world\" coordinates: it is located at $x=30$ along the first axis, and $y=15$ (within the $100x80$ domain box).\n",
     "\n",