[meta.reflection.substitute] Clarify error message in example

Fixes US 116-177 (C++26 CD).

Author: jensmaurer

source/meta.tex

@@ -6068,7 +6068,7 @@
 
 static_assert(!can_substitute(^^fn1, {^^int}));         // OK
 constexpr info r1 = substitute(^^fn1, {^^int});         // error: \tcode{fn<int>} contains an undeduced
-                                                        // placeholder type
+                                                        // placeholder type for its return type
 
 template<class T>
   auto fn2() {