doc: fix doxygen groups for k_heap

Use correct doxygen group for k_heap.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/include/kernel.h b/include/kernel.h
index d105d98..60aaf53 100644
--- a/include/kernel.h
+++ b/include/kernel.h
@@ -4423,7 +4423,7 @@
 /** @} */
 
 /**
- * @addtogroup mem_pool_apis
+ * @addtogroup heap_apis
  * @{
  */
 
@@ -4534,7 +4534,7 @@
  */
 
 /**
- * @defgroup heap_apis Heap Memory Pool APIs
+ * @defgroup heap_apis Heap APIs
  * @ingroup kernel_apis
  * @{
  */