doxygen: group name all in lowcase

This patch only updated the groups defined in documentation/0.doxygen.

Other groups will be moved into documentation/0.doxygen and updated at
that time.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

components/dfs/dfs_v1/src/dfs.c

@@ -38,7 +38,7 @@ static struct dfs_fdtable _fdtab;
 static int  fd_alloc(struct dfs_fdtable *fdt, int startfd);
 
 /**
- * @addtogroup group_DFS
+ * @addtogroup group_device_virtual_file_system
  * @{
  */
 

components/dfs/dfs_v2/src/dfs.c

@@ -139,7 +139,7 @@ void dfs_unlock(void)
     rt_mutex_release(&fslock);
 }
 
-/** @addtogroup group_DFS
+/** @addtogroup group_device_virtual_file_system
  *
  *
  *  @{