fixup: make 'direct base class relationship' a definition

Author: jensmaurer

source/classes.tex

@@ -3570,7 +3570,7 @@
 \defnadj{direct}{base class}
 for the class being defined;
 for each such \grammarterm{base-specifier},
-the corresponding direct base class relationship
+the corresponding \defnadj{direct base class}{relationship}
 is the ordered pair (\tcode{D}, \tcode{B})
 where \tcode{D} is the class being defined and
 \tcode{B} is the direct base class.