doc: groups.dox: fix mcumgr transport layers group

The group dedicated to transport layers for MCUmgr ended up being
orphan, probably due to a bad rebase in the recent past. This commit is
fixing it.

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

doc/_doxygen/groups.dox

@@ -54,9 +54,12 @@
 @brief MCUmgr
 @defgroup mcumgr MCUmgr
 @ingroup device_mgmt
+@{
+@}
 
 @brief Transport layers for MCUmgr: SMP, UART, etc.
 @defgroup mcumgr_transport Transport layers
+@ingroup mcumgr
 @{
 @}