Update biomat.md

double $ is necessary

Author: jiri-janacek

_pages/plugins/biomat.md

@@ -40,15 +40,15 @@ Example: [Synthetic contours](https://raw.githubusercontent.com/jiri-janacek/bio
  {% include img align="center" name="objects" src="https://raw.githubusercontent.com/jiri-janacek/biomat/d9ca3b6cfd867b04717b16a3debd45ef9157419e/media/objects_Spherical_extrusion.png" caption="Height of spherical extrusion" %}
 
 The binary image was extruded into 3D by union
-of balls which equatorial circles are inside the foreground $C$. The
-height of the extrusion $h$ as the function of 2D point $x$
+of balls which equatorial circles are inside the foreground $$C$$. The
+height of the extrusion $$h$$ as the function of 2D point $$x$$
 is
 
 $$
 h\left(x\right)=\max_{s}\sqrt{d\left(s,C^{c}\right)^{2}-d\left(x,s\right)^{2}}
 $$
 
-where $d$ is Euclidean distance, $C^{c}$ is the background and $s$ is the centre of the equatorial circle. 
+where $$d$$ is Euclidean distance, $$C^{c}$$ is the background and $$sS$ is the centre of the equatorial circle. 
 
  {% include img align="center" name="objects" src="https://raw.githubusercontent.com/jiri-janacek/biomat/d9ca3b6cfd867b04717b16a3debd45ef9157419e/media/objects_Spherical_extrusion_res.png" caption="Area and mean height of spherical extrusion" %}