fix(directory-structure.md): update broken link (#590)

Signed-off-by: M. Fatih Cırıt <mfc@leodrive.ai>

docs/contributing/coding-guidelines/ros-nodes/directory-structure.md

@@ -193,7 +193,7 @@ autoware_gnss_poser
     - **Rationale:** To distinguish from other source files.
   - **NOT** have `autoware_` prefix.
     - **Rationale:** To avoid verbosity.
-- See [Classes](../../class-design.md) for more details on how to construct `gnss_poser_node.hpp` and `gnss_poser_node.cpp` files.
+- See [Class design](./class-design.md) for more details on how to construct `gnss_poser_node.hpp` and `gnss_poser_node.cpp` files.
 - It is up to developer how to organize the source files under `src`.
   - **Note:** The `include` folder under `src` is optional.