Up to 3pi in ticks but only 2pi in labels

[Zitzeronion]

I honestly don't know if this per purpose, but I was a little confused. Before there were four ticks but only three tick labels. If this was correct please forgive me for this PR.

Another solution (again if it was a typo before) plot!(xticks = ([0:π:3*π;], ["0", "\\pi", "2\\pi, 3\\pi"]))