boards/mulle: fix doxygen invalid use of '@brief'

boards/mulle/include/board.h

@@ -40,7 +40,7 @@
 #define UART_STDIO_DEV      UART_DEV(1)
 
 /**
- * @brief   xtimer configuration
+ * @name    xtimer configuration
  * @{
  */
 #if 0
@@ -64,7 +64,7 @@
 /** @} */
 
 /**
- * @brief   LED pin definitions and handlers
+ * @name    LED pin definitions and handlers
  * @{
  */
 #define LED_PORT            PTC