Offset amount of archived threads to fetch by active thread count in …

…'showcase rank' command in 'showcase' extension

pcbot/exts/showcase.py

@@ -203,7 +203,8 @@ async def thread_triple(thread: discord.Thread):
                     [
                         thread
                         async for thread in channel.archived_threads(
-                            limit=amount,
+                            limit=amount
+                            - len(channel.threads),  # subtract active threads
                             before=before,
                         )
                     ],