Fix example code for islice

[ellishg]

As far as I can tell, islice('ABCDEFG', 2) should result in C D E F G, which is the same as a later example. I suspect the original author meant to use islice('ABCDEFG', 0, 2).

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[ellishg]

Actually I've realized the code in the docs are correct, but I do think the wording could be improved. I'll open a github issue instead.

[picnixz]

I don't think we need to do anything here actually. The arguments' meaning are similar to range() so it's not necessary to change the wording IMO.

[ellishg]

I realized that if there is one argument then it is stop, not start, which explains my confusion.