Fixing minor documentation error in `optika.sensors.charge_diffusion(…

…)`. (#89)

optika/sensors/_materials/_diffusion.py

@@ -113,7 +113,7 @@ def charge_diffusion(
 
     .. math::
 
-        \overline{\sigma}_\text{cd} &= \dfrac{\displaystyle \int_0^{x_{ff}} \sigma_\text{cd}(x) e^{-\alpha x} dx}
+        \overline{\sigma}_\text{cd} &= \dfrac{\displaystyle \int_0^{x_s} \sigma_\text{cd}(x) e^{-\alpha x} dx}
                                             {\displaystyle \int_0^{x_s} e^{-\alpha x} dx} \\[1mm]
                                     &= \dfrac{\displaystyle \int_0^{x_{ff}} x_{ff} \sqrt{1 - \frac{x}{x_{ff}}} e^{-\alpha x} dx}
                                             {\displaystyle \int_0^{x_s} e^{-\alpha x} dx},