Magnetometer values are also 3d vector of uT

Unless I have misunderstood something design guide should have matching the definition with [Unified Sensor Driver](https://github.com/adafruit/Adafruit_Sensor/blob/master/Adafruit_Sensor.h#L111).

Author: tuupola

docs/design_guide.rst

@@ -364,7 +364,7 @@ properties.
 +=======================+=======================+=========================================================================+
 | ``acceleration``      | (float, float, float) | x, y, z meter per second per second                                     |
 +-----------------------+-----------------------+-------------------------------------------------------------------------+
-| ``magnetic``          | float                 | micro-Tesla (uT)                                                        |
+| ``magnetic``          | (float, float, float) | micro-Tesla (uT)                                                        |
 +-----------------------+-----------------------+-------------------------------------------------------------------------+
 | ``orientation``       | (float, float, float) | x, y, z degrees                                                         |
 +-----------------------+-----------------------+-------------------------------------------------------------------------+