Update 3_drude_model.md - typo

src/3_drude_model.md

@@ -82,7 +82,7 @@ $$
 
 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 carriers either have a positive charge, or a negative mass. We will see later (chapter 17) how to interpret this.
+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 carriers either have a positive charge, or a negative mass. We will see later (chapter 17) how to interpret this.
 
 ## Exercises
 ### Exercise 1: Extracting quantities from basic Hall measurements