[structure.requirements] Add missing \tcode

Author: tkoeppe

source/lib-intro.tex

@@ -433,7 +433,7 @@
 
 \pnum
 Interface convention requirements are stated as generally as possible. Instead
-of stating ``class X has to define a member function \tcode{operator++()}'', the
+of stating ``class \tcode{X} has to define a member function \tcode{operator++()}'', the
 interface requires ``for any object \tcode{x} of class \tcode{X}, \tcode{++x} is
 defined''. That is, whether the operator is a member is unspecified.