Fix slash command flag parsing

Also removes the extra space at the end of fake message content

discord/ext/commands/bot.py

@@ -1307,7 +1307,7 @@ class BotBase(GroupMixin):
                     else:
                         prefix = param.annotation.__commands_flag_prefix__
                         delimiter = param.annotation.__commands_flag_delimiter__
-                        message.content += f"{prefix}{name} {option['value']}{delimiter}"  # type: ignore
+                        message.content += f" {prefix}{name}{delimiter}{option['value']}"  # type: ignore
                 continue
 
             option = next((o for o in command_options if o["name"] == name), None)