max31855: fix missing interesting tag in header for doc

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/max31855/max31855.h

          *
          * @return value in degrees celcius
          */
+//! [Interesting]
         double getTemp();
+//! [Interesting]
 
     private:
         mraa_spi_context m_sensor;