I was pretty confused about the buttons array as well. Since it's never used again in the code I figured it was some sort of example of how to use arrays? Odd because they don't do anything like that in the rest of the book. I completely removed both lines of code from my sketch and it complied without a problem.
Now that I looked at it again, I think you are correct. There aren't any line numbers next to the buttons array, but the notes array starts with line 1; so it's probably just there for explaining arrays (except for the fact that 'int buttons = 2' seems incorrect to me).