include: modbus: doc: move modbus doxygen group to connectivity

Modbus subsystem has to do with connectivity, it is not a device
driver class.

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

Author: kartben

include/zephyr/modbus/modbus.h

@@ -23,7 +23,7 @@
 /**
  * @brief MODBUS transport protocol API
  * @defgroup modbus MODBUS
- * @ingroup io_interfaces
+ * @ingroup connectivity
  * @{
  */