fix docstring of example

Author: alexhroom

RATapi/examples/non_polarised/DSPC_custom_XY.py

@@ -19,15 +19,15 @@ def DSPC_custom_XY():
     We can define our lipid in terms of an Area per Molecule, almost in its entirety, if we recognise that where the
     volume is known, the thickness of the layer is simply given by the layer volume / APM:
 
-    $d_{\textrm{layer}} =\frac{V_{\textrm{layer}} }{{\textrm{APM}}_{\textrm{layer}}}$.
+    .. math:: d_{\textrm{layer}} =\frac{V_{\textrm{layer}} }{{\textrm{APM}}_{\textrm{layer}}}.
 
     We can then define the Volume Fraction of this layer with a roughened Heaviside of length dlayer and a height of 1.
     Then, the total volume occupied will be given by the sum of the volume fractions across the interface. Of course,
     this does not permit any hydration, so to deal with this, we can simply scale the (full occupation) Heaviside
     functions by relevant coverage parameters. When this is correctly done, we can obtain the remaining water
     distribution as:
 
-    $${\textrm{VF}}_{\textrm{wat}} =1-\\sum_n {\textrm{VF}}_n$$
+    .. math:: {\textrm{VF}}_{\textrm{wat}} =1-\\sum_n {\textrm{VF}}_n
 
     where VFn is the Volume Fraction of the n'th layer.
     """