Add Group ID is for mapping of track event to group key $group_id

In the Troubleshooting section, modify the following (original) statement:

"Ensure that the mapping(s) handling your track events have the field for Group ID mapped to a valid value. By default, this field is mapped to the event variable context.groupId."

To the following that mentioned using of group key $group_id:

"For Mixpanel (Actions) destination that uses $group_id as the group key, ensure that the mapping(s) handling your track events have the field for Group ID mapped to a valid value. By default, this field is mapped to the event variable"context.groupId."

Author: tanjinhong72

src/connections/destinations/catalog/actions-mixpanel/index.md

@@ -133,5 +133,5 @@ If you want to confirm, you can configure the new destination to point to a diff
 
 ### Track events are not attributed to Mixpanel Groups
 
-Ensure that the mapping(s) handling your `track` events have the field for **Group ID** mapped to a valid value. By default, this field is mapped to the event variable `context.groupId`.
+For Mixpanel (Actions) destination that uses $group_id as the group key, ensure that the mapping(s) handling your `track` events have the field for **Group ID** mapped to a valid value. By default, this field is mapped to the event variable `context.groupId`.