Undo accidentally removed indentation

As this line belongs to the line above, it needs to have more indentation than the interior of the model definition.

chapters/classes.tex

@@ -1172,7 +1172,7 @@ \section{Balanced Models}\label{balanced-models}
 import Modelica.Units.SI;
 
 partial model BaseProperties
-  "Interface of medium model for all type of media"
+    "Interface of medium model for all type of media"
   parameter Boolean preferredStates = false;
   constant Integer nXi "Number of independent mass fractions";
   InputAbsolutePressure     p;