another sign error (closes #14)

docs/lecture_8.md

@@ -105,7 +105,7 @@ $$
 The result is a ladder of equally spaced levels. The bigger the contribution of $S$ inside $J$, the bigger te level spacing. Now, the magnetisation for $n$ atoms becomes:
 
 $$
-M=n\frac{\sum_{m_{J}=-J}^{J}\tilde{g}\ \mu_{\rm B}m_{J}{\rm e}^{-\beta\mu_{\rm B}m_J B}}{\sum_{m_{J}=-J}^{J}{\rm e}^{-\beta\mu_{\rm B}m_J B}}
+M=-n\frac{\sum_{m_{J}=-J}^{J}\tilde{g}\ \mu_{\rm B}m_{J}{\rm e}^{-\beta\mu_{\rm B}m_J B}}{\sum_{m_{J}=-J}^{J}{\rm e}^{-\beta\mu_{\rm B}m_J B}}
 $$
 
 The resulting susceptibility is: