[expr.prim.req.type] Add \tcode for void in comment (#8705)

Author: Andreas-Krug

source/expressions.tex

@@ -3390,7 +3390,7 @@
   typename T::inner;        // required nested member name
   typename S<T>;            // required valid\iref{temp.names} \grammarterm{template-id}; fails if \tcode{T::type} does not exist as a type
                             // to which \tcode{0} can be implicitly converted
-  typename Ref<T>;          // required alias template substitution, fails if \tcode{T} is void
+  typename Ref<T>;          // required alias template substitution, fails if \tcode{T} is \tcode{void}
   typename [:T::r1:];       // fails if \tcode{T::r1} is not a reflection of a type
   typename [:T::r2:]<int>;  // fails if \tcode{T::r2} is not a reflection of a template \tcode{Z} for which \tcode{Z<int>} is a type
 };