There is a brown burn mark on my dryer's vent
No. burn marks are not normally seen on dryer vents. If the scorched mark was caused by excessive heat from the dryer (and not by an unrelated incident) then there is a potential hazard and danger from your appliance. Without more details and photos it will be difficult to offer a specific reason, but these are some suggestions that you can check. After unplugging the dryer cord remove the exhaust duct attached to the rear of the back wall. Using a flashlight peer inside looking for critical build up of lint. Use a vacuum, regardless of what amount of lint is noted, to clean the dryer vent. Continue vacuuming the duct projecting from the back wall of the dryer. Re-install the vent and check for normal operation. If you still have any doubts to the normal and safe operation of your appliance it would be wise to have a professional inspect it and give you his opinion before using it again.