[intro.object] Add missing \tcode in comment (#8941)

Author: Andreas-Krug

source/basic.tex

@@ -3744,7 +3744,7 @@
 constexpr A v2{.i=1};   // OK, the constituent values are \tcode{\{v2.i\}}
 constexpr A v3 = []{
   A a;
-  std::start_lifetime(a.arr);   // OK, arr is now the active element of the union
+  std::start_lifetime(a.arr);   // OK, \tcode{arr} is now the active element of the union
   new (&a.arr[1]) int(1);
   a.arr[2] = 2;
   return a;