Correct a typo in comments in Group Manager class

I modify Extrracting --> Extracting in getActionSetfromString function's comment

Change-Id: Ia6bd50c1abace62529c543acc8806739ab02c7f6
Signed-off-by: Jihun Ha <jihun.ha@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/3705Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Madan Lanka <lanka.madan@samsung.com>

service/things-manager/sdk/inc/GroupManager.h

@@ -123,7 +123,7 @@ public:
     std::string getStringFromActionSet(const ActionSet *newActionSet);
 
     /**
-     * API for extrracting ActionSet class instance from an action set string.
+     * API for extracting ActionSet class instance from an action set string.
      *
      * @param desc description of an action set string
      *