Fix prompt for channel conversation summarization (slack-samples#48)

Co-authored-by: Eden Zimbelman <zim@o526.net>

Author: djjimba

listeners/listener_utils/listener_constants.py

@@ -7,8 +7,8 @@
     Mention me again in this thread so that I can help you out!
 """
 SUMMARIZE_CHANNEL_WORKFLOW = """
-User has just joined this slack channel.
-Create a quick summary of the conversaton in this channel to cath up the user.
-Don't use user names in your response.
+A user has just joined this Slack channel.
+Please create a quick summary of the conversation in this channel to help them catch up.
+Don't use user IDs or names in your response.
 """
 DEFAULT_LOADING_TEXT = "Thinking..."