Update index.md

Author: fepinheiromycin

docs/src/04_math/index.md

@@ -57,7 +57,7 @@ In this case, we are supposed to know the analytic formula of microbial growth;
 
 - **Bertalanffy**  
 
-  $$N(t) = N_0 + (N_{\text{max}} - N_0) \cdot (1 - e^{-\mu \cdot t})^{\frac{1}{\nu}}$$
+  $N(t) = N_0 + (N_{\text{max}} - N_0) \cdot (1 - e^{-\mu \cdot t})^{\frac{1}{\nu}}$
 
   where $N_0$ is the starting condition, $N_{\text{max}}$ is the total growth, $\mu$ is the growth rate, and $\nu$ is a shape constant.