[fix]: 'Count' entries in Subteam events should be int not string

The slack documentation gets it wrong: their example shows strings but looking at their
[own implementation of the types in their node sdk](https://github.com/slackapi/node-slack-sdk/blob/31c60f863c36cd74837fc9ad5d5111532451ffcd/packages/types/src/events/subteam.ts#L44
it certainly seems it should be int.

Author: nlopes

slackevents/inner_events.go

@@ -814,9 +814,9 @@ type SubteamMembersChangedEvent struct {
 	DatePreviousUpdate int      `json:"date_previous_update"`
 	DateUpdate         int64    `json:"date_update"`
 	AddedUsers         []string `json:"added_users"`
-	AddedUsersCount    string   `json:"added_users_count"`
+	AddedUsersCount    int      `json:"added_users_count"`
 	RemovedUsers       []string `json:"removed_users"`
-	RemovedUsersCount  string   `json:"removed_users_count"`
+	RemovedUsersCount  int      `json:"removed_users_count"`
 }
 
 type SubteamSelfAddedEvent struct {

slackevents/inner_events_test.go

@@ -1968,9 +1968,9 @@ func TestSubteamMembersChangedEvent(t *testing.T) {
 			"date_previous_update": 1446670362,
 			"date_update": 1624473600,
 			"added_users": ["U1234567890"],
-			"added_users_count": "3",
+			"added_users_count": 3,
 			"removed_users": ["U0987654321"],
-			"removed_users_count": "1"
+			"removed_users_count": 1
 		}
 	`)