typo fix

src/3_drude_model.md

@@ -80,7 +80,7 @@ $$
 \rho_{xy}=\frac{B}{ne}\equiv -R_{\rm H}B
 $$
 
-where $R_{\rm H}=-\frac{1}{ne}$ is the _Hall coefficient. So by measuring the Hall coefficient, we can obtain $n$, the density of free electrons in a material.
+where $R_{\rm H}=-\frac{1}{ne}$ is the *Hall coefficient*. So by measuring the Hall coefficient, we can obtain $n$, the density of free electrons in a material.
 
 While most materials have $R_{\rm H}<0$, interestingly some materials are found to have $R_{\rm H}>0$. This would imply that the charge of the carriers is positive. We will see later (chapter 17) how to interpret this.
 ## Conclusions