Fix "Class types in non-type template parameters" example.

Author: AnthonyCalandra

CPP20.md

@@ -321,7 +321,7 @@ struct foo {
   constexpr foo(int) {}
 };
 
-template <foo f>
+template <foo f = {}>
 auto get_foo() {
   return f;
 }

README.md

@@ -424,7 +424,7 @@ struct foo {
   constexpr foo(int) {}
 };
 
-template <foo f>
+template <foo f = {}>
 auto get_foo() {
   return f;
 }