[concepts.swappable] Correct example

Author: CaseyCarter

source/concepts.tex

@@ -690,16 +690,18 @@
 User code can ensure that the evaluation of \tcode{swap} calls
 is performed in an appropriate context under the various conditions as follows:
 \begin{codeblock}
+#include <cassert>
+#include <concepts>
 #include <utility>
 
-template<class T, SwappableWith<T> U>
+template<class T, std::SwappableWith<T> U>
 void value_swap(T&& t, U&& u) {
   using std::swap;
   swap(std::forward<T>(t), std::forward<U>(u)); // OK: uses ``swappable with'' conditions
                                                 // for rvalues and lvalues
 }
 
-template<Swappable T>
+template<std::Swappable T>
 void lv_swap(T& t1, T& t2) {
   using std::swap;
   swap(t1, t2);                                 // OK: uses swappable conditions for