Minor improvement in cmdline.py (#6875)

scrapy/cmdline.py

@@ -32,8 +32,9 @@ class ScrapyArgumentParser(argparse.ArgumentParser):
     def _parse_optional(
         self, arg_string: str
     ) -> tuple[argparse.Action | None, str, str | None] | None:
-        # if starts with -: it means that is a parameter not a argument
-        if arg_string[:2] == "-:":
+        # Support something like ‘-o -:json’, where ‘-:json’ is a value for
+        # ‘-o’, not another parameter.
+        if arg_string.startswith("-:"):
             return None
 
         return super()._parse_optional(arg_string)