Use 'i' and 'k' to better distinguish variables in an example

[alexwlchan]

It took me a moment of staring at this example to realise that i and j were actually different variables. They're not very different in the monospaced font!

I'm hoping that i and k will make them stand out more, and this example easier to read.

(Most letters seem to have a use already in this file, or I'd have picked an even more different pair of letters.)

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[hynek]

Thanks for improving our docs!