[Frontend][Bugfix] Fix preemption_mode -> preemption-mode for CLI arg in arg_utils.py (#5688)

Author: mgoin

vllm/engine/arg_utils.py

@@ -577,7 +577,7 @@ def add_cli_args(
                             'This should be a JSON string that will be '
                             'parsed into a dictionary.')
         parser.add_argument(
-            '--preemption_mode',
+            '--preemption-mode',
             type=str,
             default=None,
             help='If \'recompute\', the engine performs preemption by block '