use a more specific description of heat capacity

Closes #107

src/1_einstein_model.md

@@ -292,7 +292,7 @@ For low $T$, thermal fluctuations do not have enough energy to excite the vibrat
 Hence, $\langle E \rangle$ converges towards a constant value of $\hbar \omega_0/2$—the _zero-point energy_.
 Because the energy in the oscillator becomes approximately constant when $k_B T\ll\hbar \omega_0$, we already see that the heat capacity drops with temperature.
 
-Having found an expression for $\langle E \rangle$ as a function of $T$, we now calculate the heat capacity per atom $C$ explicitly by using its definition:
+Having found an expression for $\langle E \rangle$ as a function of $T$, we now calculate the heat capacity per harmonic oscillator explicitly by using its definition:
 $$
 \begin{align}
 C &\equiv \frac{d \langle E \rangle}{d T}\\