Issue 604: use the definced PETSCII values in the table.

[Rhialto]

I could not try the formatting locally so this is untested.

[dansanderson]

I support the spirit of this change but would prefer to not have a single table that skips values. Here's another attempt:

LaTeX is trying to force four columns at the top of the second page resulting in a weird gap. Not sure how to fix that, but we can deal with that later.

[dansanderson]

Implemented here: https://github.com/MEGA65/mega65-user-guide/commit/815db62347c36ec262dc15f0bbfd362ca75b61a2

Thanks very much!