[class.copy.assign] Add \tcode for const X (#8697)

Author: Andreas-Krug

source/classes.tex

@@ -1722,8 +1722,8 @@
 \tcode{X}
 only has a copy assignment operator with a non-object parameter of type
 \tcode{X\&},
-an expression of type const
-\tcode{X}
+an expression of type
+\tcode{const X}
 cannot be assigned to an object of type
 \tcode{X}.
 \begin{example}