AZURE_TWIN_UPDATE_TOPIC missing $

Please correct in mgos_azure_shadow.c the missing $ before "rid". 

After an indeterminate period of time (4-6 weeks) our IOS Hub stopped accepting updates, probably had something to do with GWV2, which was set by Azure.

#define AZURE_TWIN_UPDATE_TOPIC \
  "$iothub/twin/PATCH/properties/reported/?$rid=upd%u"