Fix AttributeError in `GenericAgentArgs.set_benchmark`: Incorrect Reference to `action_set`

[rowingchenn]

In the set_benchmark method of the GenericAgentArgs class, there is an error in the code:

The code incorrectly attempts to access GenericAgentArgs.action_set, but the GenericAgentArgs class does not have an attribute named action_set.

The correct implementation should be:

self.flags.action.action_set.demo_mode = "all_blue"