[class.copy.ctor] Add \grammarterm for using-declaration (#9267)

Author: Andreas-Krug

source/classes.tex

@@ -1606,7 +1606,7 @@
 
 \pnum
 \begin{note}
-A using-declaration in a derived class \tcode{C} that
+A \grammarterm{using-declaration} in a derived class \tcode{C} that
 names a constructor from a base class
 never suppresses the implicit declaration of
 a copy/move constructor of \tcode{C},