Fix amount of messages in log message of [p]cleanup message (#4156)

Fixes #4155

redbot/cogs/cleanup/cleanup.py

@@ -378,7 +378,7 @@ class Cleanup(commands.Cog):
         to_delete.append(ctx.message)
 
         reason = "{}({}) deleted {} messages in channel {}.".format(
-            author.name, author.id, number, channel.name
+            author.name, author.id, len(to_delete), channel.name
         )
         log.info(reason)