[module.interface] Fix outdated example

Author: lprv

source/modules.tex

@@ -382,9 +382,10 @@
 \begin{example}
 \begin{codeblock}
 export module M;
+int g;
 export namespace N {
   int x;                        // OK
-  static_assert(1 == 1);        // error: does not declare a name
+  using ::g;                    // error: \tcode{::g} has module linkage
 }
 \end{codeblock}
 \end{example}