Fix Message.system_content for role_subscription_purchase renewal type

discord/message.py

@@ -2516,10 +2516,10 @@ class Message(PartialMessage, Hashable):
             return 'Wondering who to invite?\nStart by inviting anyone who can help you build the server!'
 
         if self.type is MessageType.role_subscription_purchase and self.role_subscription is not None:
-            # TODO: figure out how the message looks like for is_renewal: true
             total_months = self.role_subscription.total_months_subscribed
             months = '1 month' if total_months == 1 else f'{total_months} months'
-            return f'{self.author.name} joined {self.role_subscription.tier_name} and has been a subscriber of {self.guild} for {months}!'
+            action = 'renewed' if self.role_subscription.is_renewal else 'joined'
+            return f'{self.author.name} {action} **{self.role_subscription.tier_name}** and has been a subscriber of {self.guild} for {months}!'
 
         if self.type is MessageType.stage_start:
             return f'{self.author.name} started **{self.content}**.'