[special] Add "direct" when defining "potentially constructed subobjects"

The definition has "non-static data members". Only the direct ones are
intended.

Author: hubert-reinterpretcast

source/classes.tex

@@ -1227,7 +1227,7 @@
 \end{itemize}
 
 \pnum
-For a class, its non-static data members, its non-virtual direct base classes,
+For a class, its direct non-static data members, its non-virtual direct base classes,
 and, if the class is not abstract\iref{class.abstract}, its virtual base
 classes are called its \term{potentially constructed subobjects}.