Added a more clear example in the cycle() docs

We've receive a lot of feedback that the examples for the cycle() tag could be better, so I figured I'd contribute one.

The previous example was potentially misleading if you were focussed on the actual numbers (0 is even, not odd! etc.), and 99% of the time you’re going to be passing loop.index0 into cycle, so I felt that should be included in the example.

I realize that this one requires a little additional setup, but I think the fact that it's a real-world example makes it worth it.