[Doc] Fix typo in the help message of '--guided-decoding-backend' (#11440)

Author: yansh97

vllm/engine/arg_utils.py

@@ -373,7 +373,7 @@ def add_cli_args(parser: FlexibleArgumentParser) -> FlexibleArgumentParser:
             choices=['outlines', 'lm-format-enforcer', 'xgrammar'],
             help='Which engine will be used for guided decoding'
             ' (JSON schema / regex etc) by default. Currently support '
-            'https://github.com/outlines-dev/outlines,'
+            'https://github.com/outlines-dev/outlines, '
             'https://github.com/mlc-ai/xgrammar, and '
             'https://github.com/noamgat/lm-format-enforcer.'
             ' Can be overridden per request via guided_decoding_backend'