Resolve "Error lecture notes Sommerfeld model: Comparison phonons vs electrons"

Closes #80 (closed)

src/4_sommerfeld_model.md

@@ -40,7 +40,7 @@ Here is a table comparing the most important properties of electrons and phonons
 |   | Phonons | Electrons |
 | - | - | - |
 | Governed by | Wave equation | Schrödinger equation |
-|  | $ d² δ\mathbf{r} / dt² = -v²∇²δ\mathbf{r}$ | $ i ħdψ/dt = -ħ²∇²ψ/2m$ |
+|  | $ d² δ\mathbf{r} / dt² = v²∇²δ\mathbf{r}$ | $ i ħdψ/dt = -ħ²∇²ψ/2m$ |
 | Dispersion relation | $ω=v \|\mathbf{k}\|$ | $ε = ħ²k²/2m$ |
 | Statistics | Bose-Einstein | Fermi-Dirac |
 | $n =$ | $1/[\exp(βE) - 1]$ | $1/[\exp(β[E - μ]) + 1]$ |