Minor forest fire fix

[jackiekazil]

From email list.....

In the demo "Forest Fire", there are two errors in "Model.py" and in "run.py", both can be solved replacing "examples.ForestFire.forest_fire.agent" by "forest_fire.agent".
Greetings and thank you for your work with MESA.

From: https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/projectmesa/LJDBpzsnE50/dAo7WYqEBAAJ

[djmitche]

This was addressed incidentally in #346, and can now be closed. I was able to run the example without issue.

[jackiekazil]

@djmitche Ty for the note!