Update interface documentation

Signed-off-by: Serban Waltter <serban.waltter@rinftech.com>
Signed-off-by: Mihai Tudor Panu <mihai.tudor.panu@intel.com>

include/interfaces/iHeartRate.hpp

@@ -37,7 +37,7 @@ namespace upm
     /**
      * Retrieve the computed heart rate
      *
-     * @return Computed heart rate
+     * @return Computed heart rate in BPM (beats per minute)
      */
     virtual int getHeartRate() = 0;
   };