Minor changes to Ch3 pivot fig/code

[trevorcampbell]

Right below the "going from long to wide with pivot" figure and the syntax figure, @joelostblom had the comment in #76

It would be nice if this figure showed the params on separate lines as with the code block below and the image for melt

Also, it is unnecessary to specify values here since all remaining columns will be used for the values parameter. This is the same reason we omitted value_vars above, so I think it would make sense to omit it here too for consistency and simplicity. I think we can add a sentence mentioning that these parameter exists for both sections