hmtrp: Add proper @defgroup documentation tag

Signed-off-by: Jon Trulson <jtrulson@ics.com>
Signed-off-by: John Van Drasek <john.r.van.drasek@intel.com>

src/hmtrp/hmtrp.h

@@ -64,6 +64,7 @@ namespace upm {
    * configuration mode by grounding the CONFIG pin on the module.
    *
    * @ingroup grove uart radio
+   * @defgroup hmtrp libupm-hmtrp
    * @snippet hmtrp.cxx Interesting
    */
   class HMTRP {