Fix typo

contents/introduction.qmd

@@ -402,7 +402,7 @@ where $V_{Ai}$ is the ion Alfvén speed.
 
 - _Electron inertial length (plasma skin depth)_ : the scale over which the electron distribution function can change substantially, or the depth in a plasma to which electromagnetic radiation can penetrate. 
 $$
-r_e = \frac{c}{\omega_{pe}} = \frac{V_{Ae}}{\omega_{ce}}
+d_e = \frac{c}{\omega_{pe}} = \frac{V_{Ae}}{\omega_{ce}}
 $$ {#eq-e-inertial-length}
 where $V_{Ae}$ is the electron Alfvén speed.