Clarify Activity parent in links example (readme) (#946)

* Clarify parent id in links scenario

Server activity had parent equal to one of the links - this is confusing, changed to default context (new trace) as likely this activity has no parent.

* Update README.md

* review comments

* fix lint

Co-authored-by: Cijo Thomas <cithomas@microsoft.com>

src/OpenTelemetry.Api/README.md

@@ -270,9 +270,22 @@ OpenTelemetry samplers chose not to sample this activity.
     activityLinks.Add(new ActivityLink(linkedContext2));
 
     var activity = activitySource.StartActivity(
-        "ActivityName",
+        "ActivityWithLinks",
         ActivityKind.Server,
-        "00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01",
+        default(ActivityContext), // this creates Activity without parent
+        initialTags,
+        activityLinks);
+    ```
+
+    In case activity has parent, pass parent's context:
+
+    ```csharp
+    var parentContext = Activity.Current != null ? Activity.Current.Context : default(ActivityContext);
+
+    var activity = activitySource.StartActivity(
+        "ActivityWithLinks",
+        ActivityKind.Server,
+        parentContext,
         initialTags,
         activityLinks);
     ```