Update 2_debye_model.md - typos

src/2_debye_model.md

@@ -190,11 +190,11 @@ $$
 
 We can trace back all the factors in the density of states to their origin:
 
-* $(L/2\pi)^3$ is the number of possible waves per unit volume in the reciprocal space.
-* $4π$ is the area of a unit sphere, the result of integration over $d \varphi$ and $dθ$.
+* $(L/2\pi)^3$ is the density of $k$-points in $k$-space.
+* $4π$ is the area of a unit sphere, the result of integrating over $d \varphi$ and $dθ$.
 * $ω^2$ is due to the area of this sphere being proportional to its squared radius.
 * $3$ is the number of possible polarizations in 3D  (two transversal, one longitudinal).
-* $v^{-3}$ is due to $ω = v|k|$.
+* $v_{\rm s}^{-3}$ is due to $ω = v_{\rm s}|k|$.
 
 
 ## Low $T$