gnrc_netif: remove premature doc group close

This adds `GNRC_NETIF_FLAGS_RAWMODE` to the `net_gnrc_netif_flags_t`
group.

sys/include/net/gnrc/netif/flags.h

@@ -128,7 +128,6 @@ enum {
  *          packet
  */
 #define GNRC_NETIF_FLAGS_MAC_RX_STARTED            (0x00008000U)
-/** @} */
 
 /**
  * @brief   Network interface is configured in raw mode