docs: fix an example in README-dev

refs #2673

Change-Id: I5910f1ecde5d91e3b2fcbecbfed37c80e476f263

README-dev.md

@@ -101,7 +101,8 @@ For example:
       return 0;
     }
     
-    }
+    } // namespace foo
+    } // namespace ndn
     
     int
     main(int argc, char** argv)