devmapper: fix typo

graphdriver/devmapper/devmapper.go

@@ -84,7 +84,7 @@ type (
 
 func (t *Task) destroy() {
 	if t != nil {
-		DmTaskDestory(t.unmanaged)
+		DmTaskDestroy(t.unmanaged)
 		runtime.SetFinalizer(t, nil)
 	}
 }

graphdriver/devmapper/devmapper_wrapper.go

@@ -148,7 +148,7 @@ type (
 )
 
 var (
-	DmTaskDestory       = dmTaskDestroyFct
+	DmTaskDestroy       = dmTaskDestroyFct
 	DmTaskCreate        = dmTaskCreateFct
 	DmTaskRun           = dmTaskRunFct
 	DmTaskSetName       = dmTaskSetNameFct