Correct the link to internal-only namespaces

Author: Break Yang

.gitignore

@@ -0,0 +1,2 @@
+.#*
+*~

README.md

@@ -92,7 +92,7 @@ them by hand.
  * Sometimes you may want to expose internal functions or
  variables just for testing purpose. In this case it is better
  to declare them in
- [internal-only namespaces](csaes/internal_only_namespaces.md).
+ [internal-only namespaces](cases/internal_only_namespaces.md).
 2. Never do **using namespace foo;**
 
  * This pollutes the namespace, and can lead to hard-to-resolve