Add a missing space in a script arg documentation (#19113)

examples/pytorch/language-modeling/run_mlm.py

@@ -72,7 +72,7 @@ class ModelArguments:
         default=None,
         metadata={
             "help": (
-                "The model checkpoint for weights initialization.Don't set if you want to train a model from scratch."
+                "The model checkpoint for weights initialization. Don't set if you want to train a model from scratch."
             )
         },
     )