fixing a typo in invoke.py (invoke-ai#2204)

Author: mauwii

scripts/invoke.py

@@ -2,7 +2,7 @@
 
 import sys
 import os
-if sys.platform == 'Darsin':
+if sys.platform == 'darwin':
     os.environ["PYTORCH_ENABLE_MPS_FALLBACK"] = "1"
 
 import ldm.invoke.CLI