[basic.scope.class] Add missing 'typename' in example (#8194)

Author: rogerorr

source/basic.tex

@@ -1544,7 +1544,7 @@
 \begin{codeblock}
 template<class D>
 struct B {
-  D::type x;            // \#1
+  typename D::type x;   // \#1
 };
 
 struct A { using type = int; };