Updated the docs for writing new training missions

Fixed: Issue 1019 Docs for 'Writing Training Missions' need to be updated with additional information about how to make the mission accessible
You said "final step" at the beginning of the paragraph but now you're saying "first step". This is a bit confusing. Also, you are implicitly directing the user to open `urls.py` here but then explicitly direct them to in line 193.

Maybe reword to "You can begin by opening `urls.py`. You'll need to tell it where the file `views.py` for your new mission..."