[Utils] Fix for MessagePredicate.lower_contained_in (#2399)

Added a missing str.lower() method when checking to see if the content is in the list.

redbot/core/utils/predicates.py

@@ -744,7 +744,7 @@ class MessagePredicate(Callable[[discord.Message], bool]):
             if not same_context(m):
                 return False
             try:
-                self.result = collection.index(m.content)
+                self.result = collection.index(m.content.lower())
             except ValueError:
                 return False
             else: