Fix a typo in the JTF dependency computation code.

It leads into a performance problem.

Author: Lifeng Lu

src/Microsoft.VisualStudio.Threading/JoinableTaskDependencyGraph.cs

@@ -760,7 +760,7 @@ private static void ComputeSelfAndDescendentOrJoinedJobsAndRemainTasks(IJoinable
                 {
                     if (reachableNodes.Add(taskOrCollection))
                     {
-                        if (remainNodes.Remove(taskOrCollection) && reachableNodes.Count == 0)
+                        if (remainNodes.Remove(taskOrCollection) && remainNodes.Count == 0)
                         {
                             // no remain task left, quit the loop earlier
                             return;