Fixup.

Author: jpellikk-arm

mbed-hal/serial_api.h

@@ -195,7 +195,7 @@ int  serial_readable(serial_t *obj);
  * @param obj The serial object
  * @return Non-zero value if any of the errors have occurred, 0 otherwise
  * @retval 0000b No errors have occurred on the serial
- * @retval 0101b RX overrrun, FIFO overflow errors
+ * @retval 0101b RX overrrun, framing errors
  * @retval 1111b All error types have occurred
  */
 int serial_error(serial_t *obj);