Match a variable name change in the tour (#198)

Author: amartini51

TSPL.docc/GuidedTour/GuidedTour.md

@@ -1845,15 +1845,15 @@ Task {
 Use task groups to structure concurrent code.
 
 ```swift
-let userIDs = await withTaskGroup(of: Int.self) { taskGroup in
+let userIDs = await withTaskGroup(of: Int.self) { group in
     for server in ["primary", "secondary", "development"] {
-        taskGroup.addTask {
+        group.addTask {
             return await fetchUserID(from: server)
         }
     }
 
     var results: [Int] = []
-    for await result in taskGroup {
+    for await result in group {
         results.append(result)
     }
     return results