audio/ble_audio.h: fix doxygen reference for ble_audio_base_bis_iter

@ref was missing, and to make all file visible to doxygen
defgroup bt_le_audio was added.

nimble/host/audio/include/audio/ble_audio.h

@@ -23,6 +23,16 @@
 #include <stdint.h>
 #include <sys/queue.h>
 
+/**
+ * @file ble_audio.h
+ *
+ * @brief Bluetooth Low Energy Audio API
+ *
+ * @defgroup bt_le_audio Bluetooth LE Audio
+ * @ingroup bt_host
+ * @{
+ */
+
 /**
  * @cond
  * Helper macros for BLE_AUDIO_BUILD_CODEC_CONFIG
@@ -644,7 +654,7 @@ int ble_audio_event_listener_unregister(struct ble_audio_event_listener *listene
  * BASE iterator
  *
  * The iterator structure used by @ref ble_audio_base_subgroup_iter and
- * @ble_audio_base_bis_iter functions to iterate the BASE Level 2 and 3 elements
+ * @ref ble_audio_base_bis_iter functions to iterate the BASE Level 2 and 3 elements
  * (Subgroups and BISes).
  * This should be used as an opaque structure and not modified manually.
  *
@@ -825,4 +835,6 @@ struct ble_audio_base {
     STAILQ_HEAD(, ble_audio_big_subgroup) subs;
 };
 
+/** @} */
+
 #endif /* H_BLE_AUDIO_ */