Formatting fixes for CWG1673.

Format code in example comment appropriately.

Author: Stefanus Du Toit

source/overloading.tex

@@ -1751,12 +1751,12 @@
 \begin{codeblock}
   struct Y { Y(int); };
   struct A { operator int(); };
-  Y y1 = A();  // error: A::operator int() is not a candidate
+  Y y1 = A();  // error: \tcode{A::operator int()} is not a candidate
 
   struct X { };
   struct B { operator X(); };
   B b;
-  X x({b});    // error: B::operator X() is not a candidate
+  X x({b});    // error: \tcode{B::operator X()} is not a candidate
 \end{codeblock}
 \exitexample