Fix doxygen section for heap documentation
Fix doxygen section for heap documentation

Signed-off-by: Fabio Utzig <66676b1ceaf93296e098708c09494361103aa635@apache.org>

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core