Fix separator in filter_log_events nextToken value. (#3914)

The old separator was present in some of the values being concatenated.

moto/logs/models.py

@@ -389,7 +389,7 @@ class LogGroup(BaseModel):
         first_index = 0
         if next_token:
             try:
-                group, stream, event_id = next_token.split("/")
+                group, stream, event_id = next_token.split("@")
                 if group != log_group_name:
                     raise ValueError()
                 first_index = (
@@ -412,7 +412,7 @@ class LogGroup(BaseModel):
         next_token = None
         if events_page and last_index < len(events):
             last_event = events_page[-1]
-            next_token = "{}/{}/{}".format(
+            next_token = "{}@{}@{}".format(
                 log_group_name, last_event["logStreamName"], last_event["eventId"]
             )