UI Tweaks

No changes have been made to the core mechanics of the demo.  I simply noticed some aspects of the interface I thought would look and work better.  

• Positive x-direction arrow now appears properly.  Apparently, the arrow symbol that I had used previously appears fine locally, but not when rendered through itch.io's server.
• The input fields' text elements are now centered in the fields instead of being left-justified.  This makes them a bit easier to read.
• The GO and RESET button sprites are now chalk smudges rather than the Unity default button sprite.

As always, please let me know if there are ways this demo can be more useful to you in the classroom.