doc: stm32: fix doxygen syntax

Missing endgroups, some accidental syntax errors/unknown commands.

include/libopencm3/stm32/l1/gpio.h

@@ -163,10 +163,10 @@ LGPL License Terms @ref lgpl_license
 /* Output type (OTx values) */
 /** @defgroup gpio_output_type GPIO Output Pin Driver Type
 @ingroup gpio_defines
-@list Push Pull
-@list Open Drain
 @{*/
+/** Push-Pull */
 #define GPIO_OTYPE_PP			0x0
+/** Open Drain */
 #define GPIO_OTYPE_OD			0x1
 /**@}*/