[temp.arg.nontype] Fix example involving string literal

P0732R2 contained some changes to this section that were missing
explicit markup; apply those changes too. Also fix missing definition of
operator<=> in class type used as type of non-type template parameter.

Author: jensmaurer

source/templates.tex

@@ -1368,18 +1368,21 @@
 \begin{example}
 
 \begin{codeblock}
-template<class T, const char* p> class X {
+template<class T, T p> class X {
   @\commentellip@
 };
 
-X<int, "Studebaker"> x1;        // error: string literal as template-argument
+X<const char*, "Studebaker"> x; // error: string literal as template-argument
 
 const char p[] = "Vivisectionist";
-X<int,p> x2;                    // OK
+X<const char*, p> y;            // OK
 
-class A { constexpr A(const char*) {} };
+class A {
+  constexpr A(const char*) {}
+  auto operator<=>(A, A) = default;
+};
 
-X<A, "Pyrophoricity"> x3;       // OK, string literal is a constructor argument to A
+X<A, "Pyrophoricity"> z;        // OK, string literal is a constructor argument to \tcode{A}
 \end{codeblock}
 \end{example}
 \end{note}