docs: fix an example in README-dev

refs #2673

Change-Id: I5910f1ecde5d91e3b2fcbecbfed37c80e476f263
diff --git a/README-dev.md b/README-dev.md
index 86f4c7b..3913124 100644
--- a/README-dev.md
+++ b/README-dev.md
@@ -101,7 +101,8 @@
       return 0;
     }
     
-    }
+    } // namespace foo
+    } // namespace ndn
     
     int
     main(int argc, char** argv)