doxygen: fix wrong ref for group examples

Fixes: 94afe6a4f1 ("doxygen: improve howto")
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

documentation/0.doxygen/example/include/groups.h

@@ -26,7 +26,7 @@
  * <a href="https://github.com/RT-Thread/rt-thread/blob/master/documentation/0.doxygen/example/include/groups.h">documentation/0.doxygen/example/include/groups.h</a>
  * for code example.
  *
- * See @ref group_doxygen_example for html output.
+ * See @ref group_doxygen_example_sub for html output.
  *
  * More information can be found in the Doxygen manual:
  * <a href="https://www.doxygen.nl/manual/grouping.html">Grouping</a>.